Blogs of Atom

Rutherford’s Nuclear Atom

2010-08-18T01:34:00.000-07:00

Scientists realized that if all atoms contain electrons but are electrically neutral, atoms must also contain an equal quantity of positive charge to balance the electrons’ negative charge. Furthermore, if electrons are indeed much less massive than even the lightest atom, then this positive charge must account for most of the mass of the atom. Thomson proposed a model by which this phenomenon could occur: He suggested that the atom was a sphere of positive charge into which the negative electrons were imbedded, like raisins in a loaf of raisin bread. In 1911 British scientist Ernest Rutherford set out to test Thomson’s proposal by firing a beam of charged particles at atoms.

Rutherford chose alpha particles for his beam. Alpha particles are heavy particles with twice the positive charge of a proton. Alpha particles are now known to be the nuclei of helium atoms, which contain two protons and two neutrons. If Thomson’s model of the atom was correct, Rutherford theorized that the electric charge and the mass of the atoms would be too spread out to significantly deflect the alpha particles. Rutherford was quite surprised to observe something very different. Most of the alpha particles did indeed change their paths by a small angle, and occasionally an alpha particle bounced back in the opposite direction. The alpha particles that bounced back must have struck something at least as heavy as themselves. This led Rutherford to propose a very different model for the atom. Instead of supposing that the positive charge and mass were spread throughout the volume of the atom, he theorized that it was concentrated in the center of the atom. Rutherford called this concentrated region of electric charge the nucleus of the atom.

In the span of 100 years, from Dalton to Rutherford, the basic ideas of atomic structure evolved from very primitive concepts of how atoms combined with one another to an understanding of the constituents of atoms—a positively charged nucleus surrounded by negatively charged electrons. The interactions between the nucleus and the electrons still required study. It was natural for physicists to model the atom, in which tiny electrons orbit a much more massive nucleus, after a familiar structure such as the solar system, in which planets orbit around a much more massive Sun. Rutherford’s model of the atom did indeed resemble a tiny solar system. The only difference between early models of the nuclear atom and the solar system was that atoms were held together by electromagnetic force, while gravitational force holds together the solar system.

The Bohr Model

Danish physicist Niels Bohr used new knowledge about the radiation emitted from atoms to develop a model of the atom significantly different from Rutherford’s model. Scientists of the 19th century discovered that when an electrical discharge passes through a small quantity of a gas in a glass tube, the atoms in the gas emit light. This radiation occurs only at certain discrete wavelengths, and different elements and compounds emit different wavelengths. Bohr, working in Rutherford’s laboratory, set out to understand the emission of radiation at these wavelengths based on the nuclear model of the atom.

Using Rutherford’s model of the atom as a miniature solar system, Bohr developed a theory by which he could predict the same wavelengths scientists had measured radiating from atoms with a single electron. However, when conceiving this theory, Bohr was forced to make some startling conclusions. He concluded that because atoms emit light only at discrete wavelengths, electrons could only orbit at certain designated radii, and light could be emitted only when an electron jumped from one of these designated orbits to another. Both of these conclusions were in disagreement with classical physics, which imposed no strict rules on the size of orbits. To make his theory work, Bohr had to propose special rules that violated the rules of classical physics. He concluded that, on the atomic scale, certain preferred states of motion were especially stable. In these states of motion an orbiting electron (contrary to the laws of electromagnetism) would not radiate energy.

At the same time that Bohr and Rutherford were developing the nuclear model of the atom, other experiments indicated similar failures of classical physics. These experiments included the emission of radiation from hot, glowing objects (called thermal radiation) and the release of electrons from metal surfaces illuminated with ultraviolet light (the photoelectric effect). Classical physics could not account for these observations, and scientists began to realize that they needed to take a new approach. They called this new approach quantum mechanics (see Quantum Theory), and they developed a mathematical basis for it in the 1920s. The laws of classical physics work perfectly well on the scale of everyday objects, but on the tiny atomic scale, the laws of quantum mechanics apply.

Quantum Theory of Atoms

The quantum mechanical view of atomic structure maintains some of Rutherford and Bohr’s ideas. The nucleus is still at the center of the atom and provides the electrical attraction that binds the electrons to the atom. Contrary to Bohr’s theory, however, the electrons do not circulate in definite planet-like orbits. The quantum-mechanical approach acknowledges the wavelike character of electrons and provides the framework for viewing the electrons as fuzzy clouds of negative charge. Electrons still have assigned states of motion, but these states of motion do not correspond to fixed orbits. Instead, they tell us something about the geometry of the electron cloud—its size and shape and whether it is spherical or bunched in lobes like a figure eight. Physicists called these states of motion orbitals. Quantum mechanics also provides the mathematical basis for understanding how atoms that join together in molecules share electrons. Nearly 100 years after Faraday’s pioneering experiments, the quantum theory confirmed that it is indeed electrical forces that are responsible for the structure of molecules.

Two of the rules of quantum theory that are most important to explaining the atom are the idea of wave-particle duality and the exclusion principle. French physicist Louis de Broglie first suggested that particles could be described as waves in 1924. In the same decade, Austrian physicist Erwin Schrödinger and German physicist Werner Heisenberg expanded de Broglie’s ideas into formal, mathematical descriptions of quantum mechanics. The exclusion principle was developed by Austrian-born American physicist Wolfgang Pauli in 1925. The Pauli exclusion principle states that no two electrons in an atom can have exactly the same characteristics.

The combination of wave-particle duality and the Pauli exclusion principle sets up the rules for filling electron orbitals in atoms. The way electrons fill up orbitals determines the number of electrons that end up in the atom’s valence shell. This in turn determines an atom’s chemical and physical properties, such as how it reacts with other atoms and how well it conducts electricity. These rules explain why atoms with similar numbers of electrons can have very different properties, and why chemical properties reappear again and again in a regular pattern among the elements.

Source: Microsoft ® Encarta ® 2009

HISTORY OF ATOMIC THEORY

2010-08-18T01:32:00.000-07:00

The work of British chemist John Dalton at the beginning of the 19th century revealed some of the first clues about the true nature of atoms. Dalton studied how quantities of different elements, such as hydrogen and oxygen, could combine to make other substances, such as water. In his book A New System of Chemical Philosophy (1808), Dalton made two assertions about atoms: (1) atoms of each element are all identical to one another but different from the atoms of all other elements, and (2) atoms of different elements can combine to form more complex substances.

Dalton’s idea that different elements had different atoms was unlike the Greek idea of atoms. The characteristics of Dalton’s atoms determined the chemical and physical properties of a substance, no matter what the substance’s form. For example, carbon atoms can form both hard diamonds and soft graphite. In the Greek theory of atoms, diamond atoms would be very different from graphite atoms. In Dalton’s theory, diamond atoms would be very similar to graphite atoms because both substances are composed of the same chemical element.

While developing his theory of atoms, Dalton observed that two elements can combine in more than one way. For example, modern scientists know that carbon monoxide (CO) and carbon dioxide (CO₂) are both compounds of carbon and oxygen. According to Dalton’s experiments, the quantities of an element needed to form different compounds are always whole-number multiples of one another. For example, two times as much oxygen is needed to form a liter of CO₂ than is needed to form a liter of CO. Dalton correctly concluded that compounds were created when atoms of pure elements joined together in fixed proportions to form units that scientists today call molecules.

States of Matter

Scientists in the early 19th century struggled in another area of atomic theory. They tried to understand how atoms of a single element could exist in solid, liquid, and gaseous forms. Scientists correctly proposed that atoms in a solid attract each other with enough force to hold the solid together, but they did not understand why the atoms of liquids and gases did not attract each other as strongly. Some scientists theorized that the forces between atoms were attractive at short distances (such as when the atoms were packed very close together to form a solid) and repulsive at larger distances (such as in a gas, where the atoms are on the average relatively far apart).

Scientists had difficulty solving the problem of states of matter because they did not adequately understand the nature of heat. Today scientists recognize that heat is a form of energy, and that different amounts of this energy in a substance lead to different states of matter. In the 19th century, however, people believed that heat was a material substance, called caloric, that could be transferred from one object to another. This explanation of heat was called the caloric theory. Dalton used the caloric theory to propose that each molecule of a gas is surrounded by caloric, which exerts a repulsive force on other molecules. According to Dalton’s theory, as a gas is heated, more caloric is added to the gas, which increases the repulsive force between the molecules. More caloric would also cause the gas to exert a greater pressure on the walls of its container, in accordance with scientists’ experiments.

This early explanation of heat and states of matter broke down when experiments in the middle of the 19th century showed that heat could change into energy of motion. The laws of physics state that the amount of energy in a system cannot increase, so scientists had to accept that heat must be energy, not a substance. This revelation required a new theory of how atoms in different states of matter behave.

Behavior of Gases

In the early 19th century Italian chemist Amedeo Avogadro made an important advance in the understanding of how atoms and molecules in a gas behave. Avogadro began his work from a theory developed by Dalton. Dalton’s theory proposed that a gaseous compound, formed by combining equal numbers of atoms of two elements, should have the same number of molecules as the atoms in one of the original elements. For example, ten atoms of the element hydrogen (H) combine with ten atoms of chlorine (Cl) to form ten gaseous hydrogen chloride (HCl) molecules.

In 1811 Avogadro developed a law of physics that seemed to contradict Dalton’s theory. Avogadro’s law states that equal volumes of different gases contain the same number of particles (atoms or molecules) if both gases are at the same temperature and pressure. In Dalton’s experiment, the volume of the original vessels containing the hydrogen or chlorine gases was the same as the volume of the vessel containing the hydrogen chloride gas. The pressures of the original hydrogen and chlorine gases were equal, but the pressure of the hydrochloric gas was twice as great as either of the original gases. According to Avogadro’s law, this doubled pressure would mean that there were twice as many hydrogen chloride gas particles than there had been chlorine particles prior to their combination.

To reconcile the results of Dalton’s experiment with his new rule, Avogadro was forced to conclude that the original vessels of hydrogen or chlorine contained only half as many particles as Dalton had thought. Dalton, however, knew the total weight of each gas in the vessels, as well as the weight of an individual atom of each gas, so he knew the total number of atoms of each gas that was present in the vessels. Avogadro reconciled the fact that there were twice as many atoms as there were particles in the vessels by proposing that gases such as hydrogen and chlorine are really made up of molecules of hydrogen and chlorine, with two atoms in each molecule. Today scientists write the chemical symbols for hydrogen and chlorine as H₂ and Cl_2, respectively, indicating that there are two atoms in each molecule. One molecule of hydrogen and one molecule of chlorine combine to form two molecules of hydrogen chlorine (H₂ + Cl₂ → 2HCl). The sample of hydrogen chloride contains twice the number of particles as either the hydrogen or chlorine because two molecules of hydrogen chloride form when a molecule of hydrogen combines with a molecule of chlorine.

Electrical Forces in Atoms

The work of Dalton and Avogadro led to a consistent view of the quantities of different gases that could be combined to form compounds, but scientists still did not understand the nature of the forces that attracted the atoms to one another in compounds and molecules. Scientists suspected that electrical forces might have something to do with that attraction, but they found it difficult to understand how electrical forces could allow two identical, neutral hydrogen atoms to attract one another to form a hydrogen molecule.

In the 1830s, British physicist Michael Faraday took the first significant step toward appreciating the importance of electrical forces in compounds. Faraday placed two electrodes connected to opposite terminals of a battery into a solution of water containing a dissolved compound. As the electric current flowed through the solution, Faraday observed that one of the elements that comprised the dissolved compound became deposited on one electrode while the other element became deposited on the other electrode. The electric current provided by the electrodes undid the coupling of atoms in the compound. Faraday also observed that the quantity of each element deposited on an electrode was directly proportional to the total quantity of current that flowed through the solution—the stronger the current, the more material became deposited on the electrode. This discovery made it clear that electrical forces must be in some way responsible for the joining of atoms in compounds.

Despite these significant discoveries, most scientists did not immediately accept that atoms as described by Dalton, Faraday, and Avogadro were responsible for the chemical and physical behavior of substances. Before the end of the 19th century, many scientists believed that all chemical and physical properties could be determined by the rules of heat, an understanding of atoms closer to that of the Greek philosophers. The development of the science of thermodynamics (the scientific study of heat) and the recognition that heat was a form of energy eliminated the role of caloric in atomic theory and made atomic theory more acceptable. The new theory of heat, called the kinetic theory, said that the atoms or molecules of a substance move faster, or gain kinetic energy, as heat energy is added to the substance. Nevertheless, a small but powerful group of scientists still did not accept the existence of atoms—they regarded atoms as convenient mathematical devices that explained the chemistry of compounds, not as real entities.

In 1905 French chemist Jean-Baptiste Perrin performed the final experiments that helped prove the atomic theory of matter. Perrin observed the irregular wiggling of pollen grains suspended in a liquid (a phenomenon called Brownian motion) and correctly explained that the wiggling was the result of atoms of the fluid colliding with the pollen grains. This experiment showed that the idea that materials were composed of real atoms in thermal motion was in fact correct.

As scientists began to accept atomic theory, researchers turned their efforts to understanding the electrical properties of the atom. Several scientists, most notably British scientist Sir William Crookes, studied the effects of sending electric current through a gas. The scientists placed a very small amount of gas in a sealed glass tube. The tube had electrodes at either end. When an electric current was applied to the gas, a stream of electrically charged particles flowed from one of the electrodes. This electrode was called the cathode, and the particles were called cathode rays.

At first scientists believed that the rays were composed of charged atoms or molecules, but experiments showed that the cathode rays could penetrate thin sheets of material, which would not be possible for a particle as large as an atom or a molecule. British physicist Sir Joseph John Thomson measured the velocity of the cathode rays and showed that they were much too fast to be atoms or molecules. No known force could accelerate a particle as heavy as an atom or a molecule to such a high speed. Thomson also measured the ratio of the charge of a cathode ray to the mass of the cathode ray. The value he measured was about 1,000 times larger than any previous measurement associated with charged atoms or molecules, indicating that within cathode rays particularly tiny masses carried relatively large amounts of charge. Thomson studied different gases and always found the same value for the charge-to-mass ratio. He concluded that he was observing a new type of particle, which carried a negative electric charge but was about a thousand times less massive than the lightest known atom. He also concluded that these particles were constituents of all atoms. Today scientists know these particles as electrons, and Thomson is credited with their discovery.

Source: Microsoft ® Encarta ® 2009

Characteristic Radiation of Atoms

2010-08-18T01:31:00.000-07:00

Atoms emit radiation when their electrons lose energy and drop down to lower orbitals, or energy states, as described in the Electron Energy Levels section above. The difference in energy between the orbitals determines the wavelength of the emitted radiation. This radiation can be in the form of visible light for outer electrons, or it can be radiation of shorter wavelengths, such as X-ray radiation, for inner electrons. Because the energies of the orbitals are strictly defined and differ from element to element, atoms of a particular element can only emit certain wavelengths of radiation. By studying the wavelengths of radiation emitted by a substance, scientists can identify the element or elements comprising the substance. For example, the outer electrons in a sodium atom emit a characteristic yellow light when they return to lower orbitals. This is why street lamps that use sodium vapor have a yellowish glow (See also Sodium-Vapor Lamp).

Chemists often use a procedure called a flame test to identify elements. In a flame test, the chemist burns a sample of the element. The heat excites the outer electrons in the element’s atoms, making the electrons jump to higher energy orbitals. When the electrons drop back down to their original orbitals, they emit light characteristic of that element. This light colors the flame and allows the chemist to identify the element.

The inner electrons of atoms also emit radiation that can help scientists identify elements. The energy it takes to boost an inner electron to a higher orbital is directly related to the positive charge of the nucleus and the pull this charge exerts on the electron. When the electron drops back to its original level, it emits the same amount of energy it absorbed, so the emitted energy is also related to the nucleus’s charge. The charge on the nucleus is equal to the atom’s atomic number.

Scientists measure the energy of the emitted radiation by measuring the radiation’s wavelength. The radiation’s energy is directly related to its wavelength, which usually resembles that of an X ray for the inner electrons. By measuring the wavelength of the radiation that an atom’s inner electron emits, scientists can identify the atom by its atomic number. Scientists used this method in the 1910s to identify the atomic number of the elements and to place the elements in their correct order in the periodic table. The method is still used today to identify particularly heavy elements (those with atomic numbers greater than 100) that are produced a few atoms at a time in large accelerators (see Transuranium Elements).

Radiation Released by Radioactivity

Atomic nuclei emit radiation when they undergo radioactive decay, as discussed in the Radioactivity section above. Nuclei usually emit radiation with very short wavelengths (and therefore high energy) when they decay. Often this radiation is in the form of gamma rays, a form of electromagnetic radiation with wavelengths even shorter than X rays. Once again, nuclei of different elements emit radiation of characteristic wavelengths. Scientists can identify nuclei by measuring this radiation. This method is especially useful in neutron activation analysis, a technique scientists use for identifying the presence of tiny amounts of elements. Scientists bombard samples that they wish to identify with neutrons. Some of the neutrons join the nuclei, making them radioactive. When the nuclei decay, they emit radiation that allows the scientists to identify the substance. Environmental scientists use neutron activation analysis in studying air and water pollution. Forensic scientists, who study evidence related to crimes, use this technique to identify gunshot residue and traces of poisons.

Particle Accelerators

Particle accelerators are devices that increase the speed of a beam of elementary particles such as protons and electrons. Scientists use the accelerated beam to study collisions between particles. The beam can collide with a target of stationary particles, or it can collide with another accelerated beam of particles moving in the opposite direction. If physicists use the nucleus of an atom as the target, the particles and radiation produced in the collision can help them learn about the nucleus. The faster the particles move, the higher the energy they contain. If collisions occur at very high energy, it is possible to create particles never before detected. In certain circumstances, energy can be converted to matter, resulting in heavier particles after the collision.

Cyclotrons and linear accelerators are two of the most important kinds of particle accelerators. In a cyclotron, a magnetic field holds a beam of charged particles in a circular path. An electric field interacts with the particles’ electric charge to give them a boost of energy and speed each time the beam goes around. In linear accelerators, charged particles move in a straight line. They receive many small boosts of energy from electric fields as they move through the accelerator.

Bombarding nuclei with beams of neutrons forces the nuclei to absorb some of the neutrons and become unstable. The unstable nuclei then decay radioactively. The way atoms decay tells scientists about the original structure of the atom. Scientists can also deduce the size and shape of nuclei from the way particles scatter from nuclei when they collide. Another use of particle accelerators is to create new and exotic isotopes, including atoms of elements with very high atomic numbers that are not found in nature.

At higher energy levels, using particles moving at much higher speeds, scientists can use accelerators to look inside protons and neutrons to examine their internal structure. At these energy levels, accelerators can produce new types of particles. Some of these particles are similar to protons or neutrons but have larger masses and are very unstable. Others have a structure similar to the pion, the particle that is exchanged between the proton and neutron as part of the strong force that binds the nucleus together. By creating new particles and studying their properties, physicists have been able to deduce their common internal structure and to classify them using the theory of quarks. High-energy collisions between one particle and another often produce hundreds of particles. Experimenters have the challenging task of identifying and measuring all of these particles, some of which exist for only the tiniest fraction of a second.

Source: Microsoft ® Encarta ® 2009

STUDYING ATOMS

2010-08-18T01:29:00.000-07:00

Physicists and chemists first learned about the properties of atoms indirectly, by studying the way that atoms join together in molecules or how atoms and molecules make up solids, liquids, and gases. Modern devices such as electron microscopes, particle traps, spectroscopes, and particle accelerators allow scientists to perform experiments on small groups of atoms and even on individual atoms. Scientists use these experiments to study the properties of atoms more directly.

Electron Microscopes

One of the most direct ways to study an object is to take its photograph. Scientists take photographs of atoms by using an electron microscope. An electron microscope imitates a normal camera, but it uses electrons instead of visible light to form an image. In photography, light reflects off of an object and is recorded on film or some other kind of detector. Taking a photograph of an atom with light is difficult because atoms are so tiny. Light, like all waves, tends to diffract, or bend around objects in its path (see Diffraction). In order to take a sharp photograph of any object, the wavelength of the light that bounces off the object must be much smaller than the size of the object. If the object is about the same size as or smaller than the light’s wavelength, the light will bend around the object and produce a fuzzy image.

Atoms are so small that even the shortest wavelengths of visible light will diffract around them. Therefore, capturing photographic images of atoms requires the use of waves that are shorter than those of visible light. X rays are a type of electromagnetic radiation like visible light, but they have very short wavelengths—much too short to be visible to human eyes. X-ray wavelengths are small enough to prevent the waves from diffracting around atoms. X rays, however, have so much energy that when they bounce off an atom, they knock electrons away from the atom. Scientists, therefore, cannot use X rays to take a picture of an atom without changing the atom. They must use a different method to get an accurate picture.

Electron microscopes provide scientists with an alternate method. Scientists shine electrons, instead of light, on an atom. As discussed in the Electrons as Waves section of this article, electrons have wavelike properties, so they can behave like light waves. The simplest type of electron microscope focuses the electrons reflected off of an object and translates the pattern formed by the reflected electrons into a visible display. Scientists have used this technique to create images of tiny insects and even individual living cells, but they have not been able to use it to make a clear image of objects smaller than about 10 nanometers (abbreviated nm), or 1 × 10^-8 m (4 × 10^-7 in).

To get to the level of individual atoms, scientists must use a more powerful type of electron microscope called a scanning tunneling microscope (STM). An STM uses a tiny probe, the tip of which can be as small as a single atom, to scan an object. An STM takes advantage of another wavelike property of electrons called tunneling. Tunneling allows electrons emitted from the probe of the microscope to penetrate, or tunnel into, the surface of the object being examined. The rate at which the electrons tunnel from the probe to the surface is related to the distance between the probe and the surface. These moving electrons generate a tiny electric current that the STM measures. The STM constantly adjusts the height of the probe to keep the current constant. By tracking how the height of the probe changes as the probe moves over the surface, scientists can get a detailed map of the surface. The map can be so detailed that individual atoms on the surface are visible.

Particle Traps

Studying single atoms or small samples of atoms can help scientists understand atomic structure. However, all atoms, even atoms that are part of a solid material, are constantly in motion. This constant motion makes them difficult to examine. To study single atoms, scientists must slow the atoms down and confine them to one place. Scientists can slow and trap atoms using devices called particle traps.

Slowing down atoms is actually the same as cooling them. This is because an atom’s rate of motion is directly related to its temperature. Atoms that are moving very quickly cause a substance to have a high temperature. Atoms moving more slowly create a lower temperature. Scientists therefore build traps that cool atoms down to a very low temperature.

Several different types of particle traps exist. Some traps are designed to slow down ions, while others are designed to slow electrically neutral atoms. Traps for ions often use electric and magnetic fields to influence the movement of the particle, confining it in a small space or slowing it down. Traps for neutral atoms often use lasers, beams of light in which the light waves are uniform and consistent. Light has no mass, but it moves so quickly that it does have momentum. This property allows the light to affect other particles, or “bump” into them. When laser light collides with atoms, the momentum of the light forces the atoms to change speed and direction.

Scientists use trapped and cooled atoms for a variety of experiments, including those that precisely measure the properties of individual atoms and those in which scientists construct extremely accurate atomic clocks. Atomic clocks keep track of time by counting waves of radiation emitted by atoms in traps inside the clock. Because the traps hold the atoms at low temperatures, the mechanisms inside the clock can exercise more control over the atom, reducing the possibility of error. Scientists can also use isolated atoms to measure the force of gravity in an area with extreme accuracy. These measurements are useful in oil exploration, among other things. A deposit of oil or other substance beneath Earth’s surface has a different density than the material surrounding it. The strength of the pull of gravity in an area depends on the density of material in the area, so these changes in density produce changes in the local strength of gravity. Advances in the manipulation of atoms have also raised the possibility of using atoms to etch electronic circuits. This would help make the circuits smaller and thereby allow more circuits to fit in a tinier area.

In 1995 American physicists used particle traps to cool a sample of rubidium atoms to a temperature near absolute zero (-273°C, or –459°F). Absolute zero is the temperature at which all motion stops. When the scientists cooled the rubidium atoms to such a low temperature, the atoms slowed almost to a stop. The scientists knew that the momentum of the atoms, which is related to their speed, was close to zero. At this point, a special rule of quantum physics, called the uncertainty principle, greatly affected the positions of the atoms. This rule states that the momentum and position of a particle both cannot have precise values at the same time. The scientists had a fairly precise value for the atom’s momentum (nearly zero), so the positions of the atoms became very imprecise. The position of each atom could be described as a large, fuzzy cloud of probability. The atoms were very close together in the trap, so the probability clouds of many atoms overlapped one another. It was impossible for the scientists to tell where one atom ended and another began. In effect, the atoms formed one huge particle. This new state of matter is called a Bose-Einstein condensate.

Spectroscopes

Spectroscopy is the study of the radiation, or energy, that atoms, ions, molecules, and atomic nuclei emit. This emitted energy is usually in the form of electromagnetic radiation—vibrating electric and magnetic waves. Electromagnetic waves can have a variety of wavelengths, including those of visible light. X rays, ultraviolet radiation, and infrared radiation are also forms of electromagnetic radiation. Scientists use spectroscopes to measure this emitted radiation.

Source: Microsoft ® Encarta ® 2009

Atomic Properties

2010-08-18T01:28:00.000-07:00

The atom’s electron cloud, that is, the arrangement of electrons around an atom, determines most of the atom’s physical and chemical properties. Scientists can therefore predict how atoms will interact with other atoms by studying their electron clouds. The electrons in the outermost shell largely determine the chemical properties of an atom. If this shell is full, meaning all the orbitals in the shell have two electrons, then the atom is stable, and it won’t react readily with other atoms. If the shell is not full, the atom will chemically react with other atoms, exchanging or sharing electrons in order to fill its outer shell. Atoms bond with other atoms to fill their outer shells because it requires less energy to exist in this bonded state. Atoms always seek to exist in the lowest energy state possible.

Valence Shells

Physicists call the outer shell of an atom its valence shell. The valence shell determines the atom’s chemical behavior, or how it reacts with other elements. The fullness of an atom’s valence shell affects how the atom reacts with other atoms. Atoms with valence shells that are completely full are not likely to interact with other atoms. Six gaseous elements—helium, neon, argon, krypton, xenon, and radon—have full valence shells. These six elements are often called the noble gases because they do not normally form compounds with other elements. The noble gases are chemically inert because their atoms are in a state of low energy. A full valence shell, like that of atoms of noble gases, provides the lowest and most stable energy for an atom.

Atoms that do not have a full valence shell try to lower their energy by filling up their valence shell. They can do this in several ways: Two atoms can share electrons to complete the valence shell of both atoms, an atom can shed or take on electrons to create a full valence shell, or a large number of atoms can share a common pool of electrons to complete their valence shells.

Covalent Bonds

When two atoms share a pair of electrons, they form a covalent bond. When atoms bond covalently, they form molecules. A molecule can be made up of two or more atoms, all joined with covalent bonds. Each atom can share its electrons with one or more other atoms. Some molecules contain chains of thousands of covalently bonded atoms.

Carbon is an important example of an element that readily forms covalent bonds. Carbon has a total of six electrons. Two of the electrons fill up the first orbital, the 1s orbital, which is the only orbital in the first shell. The rest of the electrons partially fill carbon’s valence shell. Two fill up the next orbital, the 2s orbital, which forms the 2s subshell. Carbon’s valence shell still has the 2p subshell, containing three p-orbitals. The two remaining electrons each fill half of the two orbitals in the 2p subshell. The carbon atom thus has two half-full orbitals and one empty orbital in its valence shell. A carbon atom fills its valence shell by sharing electrons with other atoms, creating covalent bonds. The carbon atom can bond with other atoms through any of the three unfilled orbitals in its valence shell. The three available orbitals in carbon’s valence shell enable carbon to bond with other atoms in many different ways. This flexibility allows carbon to form a great variety of molecules, which can have a similarly great variety of geometrical shapes. This diversity of carbon-based molecules is responsible for the importance of carbon in molecules that form the basis for living things (see Organic Chemistry).

Ionic Bonds

Atoms can also lose or gain electrons to complete their valence shell. An atom will tend to lose electrons if it has just a few electrons in its valence shell. After losing the electrons, the next lower shell, which is full, becomes its valence shell. An atom will tend to steal electrons away from other atoms if it only needs a few more electrons to complete the shell. Losing or gaining electrons gives an atom a net electric charge because the number of electrons in the atom is no longer the same as the number of protons. Atoms with net electric charge are called ions. Scientists call atoms with a net positive electric charge cations (pronounced CAT-eye-uhns) and atoms with a net negative electric charge anions (pronounced AN-eye-uhns).

The oppositely charged cations and anions are attracted to each other by electromagnetic force and form ionic bonds. When these ions come together, they form crystals. A crystal is a solid material made up of repeating patterns of atoms. Alternating positive and negative ions build up into a solid lattice, or framework. Crystals are also called ionic compounds, or salts.

The element sodium is an example of an atom that has a single electron in its valence shell. It will easily lose this electron and become a cation. Chlorine atoms are just one electron away from completing their valence shell. They will tend to steal an electron away from another atom, forming an anion. When sodium and chlorine atoms come together, the sodium atoms readily give up their outer electron to the chlorine atoms. The oppositely charged ions bond with each other to form the crystal known as sodium chloride, or table salt. See also Chemical Reaction.

Metallic Bonds

Atoms can complete their valence shells in a third way: by bonding together in such a way so that all the atoms in the substance share each other’s outer electrons. This is the way metallic elements bond and fill their valence shells. Metals form crystal lattice structures similar to salts, but the outer electrons in their atoms do not belong to any atom in particular. Instead, the outer electrons belong to all the atoms in the crystal, and they are free to move throughout the crystal. This property makes metals good conductors of electricity.

The Periodic Table

The organization of the periodic table reflects the way elements fill their orbitals with electrons. Scientists first developed this chart by grouping together elements that behave similarly in order of increasing atomic number. Scientists eventually realized that the chemical and physical behavior of elements was dependant on the electron clouds of the atoms of each element. The periodic table does not have a simple rectangular shape. Each column lists elements that share chemical properties, properties that depend on the arrangement of electrons in the orbitals of atoms. These elements have the same number of electrons in their valence shells. Different numbers of elements have similar valence shells, so the columns of the periodic table differ in height. The noble gases are all located in the rightmost column of the periodic table, labeled column 18 in Encarta’s periodic table. The noble gases all have full valence shells and are extremely stable. The column labeled 11 holds the elements copper, silver, and gold. These elements are metals that have partially filled valence shells and conduct electricity well.

Electron Energy Levels

Each electron in an atom has a particular energy. This energy depends on the electron’s speed, the presence of other electrons, the electron’s distance from the nucleus, and the positive charge of the nucleus. For atoms with more than one electron, calculating the energy of each electron becomes too complicated to be practical. However, the order and relative energies of electrons follows the order of the electron orbitals, as discussed in the Electron Orbital and Shell section of this article. Physicists call the energy an electron has in a particular orbital the energy state of the electron. For example, the 1s orbital holds the two electrons with the lowest possible energies in an atom. These electrons are in the lowest energy state of any electrons in the atom.

When an atom gains or loses energy, it does so by adding energy to, or removing energy from, its electrons. This change in energy causes the electrons to move from one orbital, or allowed energy state, to another. Under ordinary conditions, all electrons in an atom are in their lowest possible energy states, given that only two electrons can occupy each orbital. Atoms gain energy by absorbing it from light or from a collision with another particle, or they gain it by entering an electric or magnetic field. When an atom absorbs energy, one or more of its electrons moves to a higher, or more energetic, orbital. Usually atoms can only hold energy for a very short amount of time—typically 1 × 10^-12 seconds or less. When electrons drop back down to their original energy states, they release their extra energy in the form of a photon (a packet of radiation). Sometimes this radiation is in the form of visible light. The light emitted by a fluorescent lamp is an example of this process.

The outer electrons in an atom are easier to move to higher orbitals than the electrons in lower orbitals. The inner electrons require more energy to move because they are closer to the nucleus and therefore experience a stronger electromagnetic pull toward the nucleus than the outer electrons. When an inner electron absorbs energy and then falls back down, the photon it emits has more energy than the photon an outer electron would emit. The emitted energy relates directly to the wavelength of the photon. Photons with more energy are made of radiation with a shorter wavelength. When inner electrons drop down, they emit high-energy radiation, in the range of an X ray. X rays have much shorter wavelengths than visible light. When outer electrons drop down, they emit light with longer wavelengths, in the range of visible light.

Source: Microsoft ® Encarta ® 2009

THE QUANTUM ATOM

2010-08-18T01:26:00.000-07:00

Scientists of the early 20th century found they could not explain the behavior of atoms using their current knowledge of matter. They had to develop a new view of matter and energy to accurately describe how atoms behaved. They called this theory quantum theory, or quantum mechanics. Quantum theory describes matter as acting both as a particle and as a wave. In the visible objects encountered in everyday life, the wavelike nature of matter is too small to be apparent. Wavelike nature becomes important, however, in microscopic particles such as electrons. As we have discussed, electrons in atoms behave like waves. They exist as a fuzzy cloud of negative charge around the nucleus, instead of as a particle located at a single point.

Wave Behavior

In order to understand the quantum model of the atom, we must know some basic facts about waves. Waves are vibrations that repeat regularly over and over again. A familiar example of waves occurs when one end of a rope is tied to a fixed object and someone moves the other end up and down. This action creates waves that travel along the rope. The highest point that the rope reaches is called the crest of the wave. The lowest point is called the trough of the wave. Troughs and crests follow each other in a regular sequence. The distance from one trough to the next trough, or from one crest to the next crest, is called a wavelength. The number of wavelengths that pass a certain point in a given amount of time is called the wave’s frequency.

In physics, the word wave usually means the entire pattern, which may consist of many individual troughs and crests. For example, when the person holding the loose end of the rope moves it up and down very fast, many troughs and crests occupy the rope at once. A physicist would use the word wave to describe the entire set of troughs and crests on the rope.

When two waves meet each other, they merge in a process called interference. Interference creates a new wave pattern. If two waves with the same wavelength and frequency come together, the resulting pattern depends on the relative position of the waves’ crests. If the crests and troughs of the two waves coincide, the waves are said to be in phase. Waves in phase with each other will merge to produce higher crests and lower troughs. Physicists call this type of interference constructive interference.

Sometimes waves with the same wavelength and frequency are out of phase, meaning they meet in such a way that their respective crests and troughs do not coincide. In these cases the waves produce destructive interference. If two identical waves are exactly half a wavelength out of phase, the crests of one wave line up with the troughs of the other. These waves cancel each other out completely, and no wave will appear. If two waves meet that are not exactly in phase and not exactly one-half wavelength out of phase, they will interfere constructively in some places and destructively in others, producing a complicated new wave. See also Wave Motion.

Electrons as Waves

Electrons behave as both particles and waves in atoms. This characteristic is called wave-particle duality. Wave-particle duality actually affects all particles and collections of particles, including protons, neutrons, and atoms themselves. But in terms of the structure of the atom, the wavelike nature of the electron is the most important.

Electron Orbitals and Shells

Physicists call the region of space an electron occupies in an atom the electron’s orbital. Similar orbitals constitute groups called shells. The electrons in the orbitals of a particular shell have similar levels of energy. This energy is in the form of both kinetic energy and potential energy. Lower shells are close to the nucleus and higher shells are farther from the nucleus. Electrons occupying orbitals in higher shells generally have more energy than electrons occupying orbitals in lower shells.

Differences Between Orbitals

The wavelike nature of electrons sets boundaries for their possible locations and determines what shape their orbital, or cloud of probability, will form. Orbitals differ from each other in size, angular momentum, and magnetic properties. In general, angular momentum is the energy an object contains based on how fast the object is revolving, the object’s mass, and the object’s distance from the axis around which it is revolving. The angular momentum of a whirling ball tied to a string, for example, would be greater if the ball was heavier, the string was longer, or the whirling was faster. In atoms, the angular momentum of an electron orbital depends on the size and shape of the orbital. Orbitals with the same size and shape all have the same angular momentum. Some orbitals, however, can differ in shape but still have the same angular momentum. The magnetic properties of an orbital describe how it would behave in a magnetic field. Magnetic properties also depend on the size and shape of the orbital, as well as on the orbital’s orientation in space.

The orbitals in an atom must occur at certain distances from the nucleus to create a stable atom. At these distances, the orbitals allow the electron wave to complete one or more half-wavelengths (y, 1, 1y, 2, 2y, and so on) as it travels around the nucleus. The electron wave can then double back on itself and constructively interfere with itself in a way that reinforces the wave. Any other distance would cause the electron to interfere with its own wave in an unpredictable and unstable way, creating an unstable atom.

Principal and Secondary Quantum Numbers

Physicists call the number of half-wavelengths that an orbital allows the orbital’s principal quantum number (abbreviated n). In general, this number determines the size of the orbital. Larger orbitals allow more half-wavelengths and therefore have higher principal quantum numbers. The orbital that allows one half-wavelength has a principal quantum number of one. Only one orbital allows one half-wavelength. More than one orbital can allow two or more half-wavelengths. These orbitals may have the same principal quantum number, but they differ from each other in their angular momentum and their magnetic properties. The orbitals that allow one wavelength have a principal quantum number of 2 (n = 2), the orbitals that allow one and a half wavelengths have a principal quantum number of 3 (n = 3), and so on. The set of orbitals with the same principal quantum number make up a shell.

Physicists use a second number to describe the angular momentum of an orbital. This number is called the orbital’s secondary quantum number, or its angular momentum quantum number (abbreviated l). The number of possible values an orbital can have for its angular momentum is one less than the number of half-wavelengths it allows. This means that an orbital with a principal quantum number of n can have n-1 possible values for its secondary quantum number.

Physicists customarily use letters to indicate orbitals with certain secondary quantum numbers. In order of increasing angular momentum, the orbitals with the six lowest secondary quantum numbers are indicated by the letters s, p, d, f, g, and h. The letter s corresponds to the secondary quantum number 0, the letter p corresponds to the secondary quantum number 1, and so on. In general, the angular momentum of an orbital depends on its shape. An s-orbital, with a secondary quantum number of 0, is spherical. A p-orbital, with a secondary quantum number of 1, resembles two hemispheres, facing one another. The possible combinations of principal and secondary quantum numbers for the first five shells are listed below.

Subshells

More than one orbital can allow the same number of half-wavelengths and have the same angular momentum. Physicists call orbitals in a shell that all have the same angular momentum a subshell. They designate a subshell with the subshell’s principal and secondary quantum numbers. For example, the 1s subshell is the group of orbitals in the first shell with an angular momentum described by the letter s. The 2p subshell is the group of orbitals in the second shell with an angular momentum described by the letter p.

Orbitals within a subshell differ from each other in their magnetic properties. The magnetic properties of an orbital depend on its shape and orientation in space. For example, a p-orbital can have three different orientations in space: one situated up and down, one from side to side, and a third from front to back.

Magnetic Quantum Number and Spin

Physicists describe the magnetic properties of an orbital with a third quantum number called the orbital’s magnetic quantum number (abbreviated m). The magnetic quantum number determines how orbitals with the same size and angular momentum are oriented in space. An orbital’s magnetic quantum number can only have whole number values ranging from the value of the orbital’s secondary quantum number down to the negative value of the secondary quantum number. A p-orbital, for example, has a secondary quantum number of 1 (l = 1), so the magnetic quantum number has three possible values: +1, 0, and -1. This means the p-orbital has three possible orientations in space. An s-orbital has a secondary quantum number of 0 (l = 0), so the magnetic quantum number has only one possibility: 0. This orbital is a sphere, and a sphere can only have one orientation in space. For a d-orbital, the secondary quantum number is 2 (l = 2), so the magnetic quantum number has five possible values: -2, -1, 0, +1, and +2. A d-orbital has four possible orientations in space, as well as a fifth orbital that differs in shape from the other four. Together, the principal, secondary, and magnetic quantum numbers specify a particular orbital in an atom.

Electrons are a type of particle known as a fermion. Austrian-American physicist Wolfgang Pauli discovered that no two fermions can have the exact same quantum numbers. This principle is called the Pauli exclusion principle, which states that two or more identical electrons cannot occupy the same orbital in an atom. Scientists know, however, that each orbital can hold two electrons. Electrons have another property, called spin, that differentiates the two electrons in each orbital. An electron’s spin has two possible values: +y (called spin-up) or -y (called spin-down). These two possible values mean that two electrons can occupy the same orbital, as long as their spins are different. Physicists call spin the fourth quantum number of an electron orbital (abbreviated m_s). Spin, in addition to the other three quantum numbers, uniquely describes a particular electron’s orbital.

Filling Orbitals

When electrons collect around an atom’s nucleus, they fill up orbitals in a definite pattern. They seek the first available orbital that takes the least amount of energy to occupy. Generally, it takes more energy to occupy orbitals with higher quantum numbers. It takes the same energy to occupy all the orbitals in a subshell. The lowest energy orbital is the one closest to the nucleus. It has a principal quantum number of 1, a secondary quantum number of 0, and a magnetic quantum number of 0. The first two electrons—with opposite spins—occupy this orbital.

If an atom has more than two electrons, the electrons begin filling orbitals in the next subshell with one electron each until all the orbitals in the subshell have one electron. The electrons that are left then go back and fill each orbital in the subshell with a second electron with opposite spin. They follow this order because it takes less energy to add an electron to an empty orbital than to complete a pair of electrons in an orbital. The electrons fill all the subshells in a shell, then go on to the next shell. As the subshells and shells increase, the order of energy for orbitals becomes more complicated. For example, it takes slightly less energy to occupy the s-subshell in the fourth shell than it does to occupy the d-subshell in the third shell. Electrons will therefore fill the orbitals in the 4s subshell before they fill the orbitals in the 3d subshell, even though the 3d subshell is in a lower shell.

Source: Microsoft ® Encarta ® 2009

FORCES ACTING INSIDE ATOMS

2010-08-18T01:24:00.000-07:00

In physics, a force is a push or pull on an object. There are four fundamental forces, three of which—the electromagnetic force, the strong force, and the weak force—are involved in keeping stable atoms in one piece and determining how unstable atoms will decay. The electromagnetic force keeps electrons attached to their atom. The strong force holds the protons and neutrons together in the nucleus. The weak force governs how atoms decay when they have excess protons or neutrons. The fourth fundamental force, gravity, only becomes apparent with objects much larger than subatomic particles.

Electromagnetic Force

The most familiar of the forces at work inside the atom is the electromagnetic force. This is the same force that causes people’s hair to stick to a brush or comb when they have a buildup of static electricity. The electromagnetic force causes opposite electric charges to attract each other. Because of this force, the negatively charged electrons in an atom are attracted to the positively charged protons in the atom’s nucleus. This force of attraction binds the electrons to the atom. The electromagnetic force becomes stronger as the distance between charges becomes smaller. This property usually causes oppositely charged particles to come as close to each other as possible. For many years, scientists wondered why electrons didn’t just spiral into the nucleus of an atom, getting as close as possible to the protons. Physicists eventually learned that particles as small as electrons can behave like waves, and this property keeps electrons at set distances from the atom’s nucleus. The wavelike nature of electrons is discussed below in the Quantum Atom section of this article.

The electromagnetic force also causes like charges to repel each other. The negatively charged electrons repel one another and tend to move far apart from each other, but the positively charged nucleus exerts enough electromagnetic force to keep the electrons attached to the atom. Protons in the nucleus also repel one other, but, as described below, the strong force overcomes the electromagnetic force in the nucleus to hold the protons together.

Strong Force

Protons and neutrons in the nuclei of atoms are held together by the strong force. This force must overcome the electromagnetic force of repulsion the protons in a nucleus exert on one another. The strong force that occurs between protons alone, however, is not enough to hold them together. Other particles that add to the strong force, but not to the electromagnetic force, must be present to make a nucleus stable. The particles that provide this additional force are neutrons. Neutrons add to the strong force of attraction but have no electric charge and so do not increase the electromagnetic repulsion.

Range of the Strong Force

The strong force only operates at very short range—about 2 femtometers (abbreviated fm), or 2 × 10^-15 m (8 × 10^-14 in). Physicists also use the word fermi (also abbreviated fm) for this unit in honor of Italian-born American physicist Enrico Fermi. The short-range property of the strong force makes it very different from the electromagnetic and gravitational forces. These latter forces become weaker as distance increases, but they continue to affect objects millions of light-years away from each other. Conversely, the strong force has such limited range that not even all protons and neutrons in the same nucleus feel each other’s strong force. Because the diameter of even a small nucleus is about 5 to 6 fm, protons and neutrons on opposite sides of a nucleus only feel the strong force from their nearest neighbors.

The strong force differs from electromagnetic and gravitational forces in another important way—the way it changes with distance. Electromagnetic and gravitational forces of attraction increase as particles move closer to one another, no matter how close the particles get. This increase causes particles to move as close together as possible. The strong force, on the other hand, remains roughly constant as protons and neutrons move closer together than about 2 fm. If the particles are forced much closer together, the attractive nuclear force suddenly turns repulsive. This property causes nuclei to form with the same average spacing—about 2 fm—between the protons and neutrons, no matter how many protons and neutrons there are in the nucleus.

The unique nature of the strong force determines the relative number of protons and neutrons in the nucleus. If a nucleus has too many protons, the strong force cannot overcome the electromagnetic repulsion of the protons. If the nucleus has too many neutrons, the excess strong force tries to crowd the protons and neutrons too close together. Most stable atomic nuclei fall between these extremes. Lighter nuclei, such as carbon-12 and oxygen-16, are made up of 50 percent protons and 50 percent neutrons. More massive nuclei, such as bismuth-209, contain about 40 percent protons and 60 percent neutrons.

Pions

Particle physicists explain the behavior of the strong force by introducing another type of particle, called a pion. Protons and neutrons interact in the nucleus by exchanging pions. Exchanging pions pulls protons and neutrons together. The process is similar to two people having a game of catch with a heavy ball, but with each person attached to the ball by a spring. As one person throws the ball to the other, the spring pulls the thrower toward the ball. If the players exchange the ball rapidly enough, the ball and springs become just a blur to an observer, and it appears as if the two throwers are simply pulled toward one another. This is what occurs in the nuclei of atoms. The protons and neutrons in the nucleus are the people, pions act as the ball, and the strong force acts as the springs holding everything together.

Pions in the nucleus exist only for the briefest instant of time, no more than 1 × 10^-23 seconds, but even during their short existence they can provide the attraction that holds the nucleus together. Pions can also exist as independent particles outside of the nucleus of an atom. Scientists have created them by striking high-speed protons against a target. Even though the free pions also live only for a short period of time (about 1 × 10^-8 seconds), scientists have been able study their properties.

Weak Force

The weak force lives up to its name—it is much weaker than the electromagnetic and strong forces. Like the strong force, it only acts over a short distance, about .01 fm. Unlike these other forces, however, the weak force affects all the particles in an atom. The electromagnetic force only affects the electrons and protons, and the strong force only affects the protons and neutrons. When a nucleus has too many protons to hold together or so many neutrons that the strong force squeezes too tightly, the weak force actually changes one type of particle into another. When an atom undergoes one type of decay, for example, the weak force causes a neutron to change into a proton, an electron, and an electron antineutrino. The total electric charge and the total energy of the particles remain the same before and after the change.

Source: Microsoft ® Encarta ® 2009

PROPERTIES OF ATOMS

2010-08-18T01:22:00.000-07:00

Atoms have several properties that help distinguish one type of atom from another and determine how atoms change under certain conditions.

Atomic Number

Each element has a unique number of protons in its atoms. This number is called the atomic number (abbreviated Z). Because atoms are normally electrically neutral, the atomic number also specifies how many electrons an atom will have. The number of electrons, in turn, determines many of the chemical and physical properties of the atom. The lightest atom, hydrogen, has an atomic number equal to one, contains one proton, and (if electrically neutral) one electron. The most massive stable atom found in nature is bismuth (Z = 83). More massive unstable atoms also exist in nature, but they break apart and change into other atoms over time. Scientists have produced even more massive unstable elements in laboratories.

Mass Number

The total number of protons and neutrons in the nucleus of an atom is the mass number of the atom (abbreviated A). The mass number of an atom is an approximation of the mass of the atom. The electrons contribute very little mass to the atom, so they are not included in the mass number. A stable helium atom can have a mass number equal to three (two protons plus one neutron) or equal to four (two protons plus two neutrons). Bismuth, with 83 protons, requires 126 neutrons for stability, so its mass number is 209 (83 protons plus 126 neutrons).

Atomic Mass and Weight

Scientists usually measure the mass of an atom in terms of a unit called the atomic mass unit (abbreviated amu). They define an amu as exactly 1/12 the mass of an atom of carbon with six protons and six neutrons. On this scale, the mass of a proton is 1.00728 amu and the mass of a neutron is 1.00866 amu. The mass of an atom measured in amu is nearly equal to its mass number.

Scientists can use a device called a mass spectrometer to measure atomic mass. A mass spectrometer removes one or more electrons from an atom. The electrons are so light that removing them hardly changes the mass of the atom at all. The spectrometer then sends the atom through a magnetic field, a region of space that exerts a force on magnetic or electrically charged particles. Because of the missing electrons, the atom has more protons than electrons and hence a net positive charge. The magnetic field bends the path of the positively charged atom as it moves through the field. The amount of bending depends on the atom’s mass. Lighter atoms will be affected more strongly than heavier atoms. By measuring how much the atom’s path curves, a scientist can determine the atom’s mass.

The atomic mass of an atom, which depends on the number of protons and neutrons present, also relates to the atomic weight of an element. Weight usually refers to the force of gravity on an object, but atomic weight is really just another way to express mass. An element’s atomic weight is given in grams. It represents the mass of one mole (6.02 × 10²³ atoms) of that element. Numerically, the atomic weight and the atomic mass of an element are the same, but the first is expressed in grams and the second is in atomic mass units. So, the atomic weight of hydrogen is 1 gram and the atomic mass of hydrogen is 1 amu.

Isotopes

Atoms of the same element that differ in mass number are called isotopes. Since all atoms of a given element have the same number of protons in their nucleus, isotopes must have different numbers of neutrons. Helium, for example, has an atomic number of 2 because of the two protons in its nucleus. But helium has two stable isotopes—one with one neutron in the nucleus and a mass number equal to three and another with two neutrons and a mass number equal to four.

Scientists attach the mass number to an element’s name to differentiate between isotopes. Under this convention, helium with a mass number of three is called helium-3, and helium with a mass number of four is called helium-4. Helium in its natural form on Earth is a mixture of these two isotopes. The percentage of each isotope found in nature is called the isotope’s isotopic abundance. The isotopic abundance of helium-3 is very small, only 0.00014 percent, while the abundance of helium-4 is 99.99986 percent. This means that only about one of every 1 million helium atoms is helium-3, and the rest are all helium-4. Bismuth has only one naturally occurring stable isotope, bismuth-209. Bismuth-209’s isotopic abundance is therefore 100 percent. The element with the largest number of stable isotopes found in nature is tin, which has ten stable isotopes.

All elements also have unstable isotopes, which are more susceptible to breaking down, or decaying, than are the other isotopes of an element. When atoms decay, the number of protons in their nucleus changes. Since the number of protons in the nucleus of an atom determines what element that atom belongs to, this decay changes one element into another. Different isotopes decay at different rates. One way to measure the decay rate of an isotope is to find its half-life. An isotope’s half-life is the time that passes until half of a sample of an isotope has decayed.

The various isotopes of a given element have nearly identical chemical properties and many similar physical properties. They differ, of course, in their mass. The mass of a helium-3 atom, for example, is 3.016 amu, while the mass of a helium-4 atom is 4.003 amu.

Usually scientists do not specify the atomic weight of an element in terms of one isotope or another. Instead, they express atomic weight as an average of all of the naturally occurring isotopes of the element, taking into account the isotopic abundance of each. For example, the element copper has two naturally occurring isotopes: copper-63, with a mass of 62.930 amu and an isotopic abundance of 69.2 percent, and copper-65, with a mass of 64.928 amu and an abundance of 30.8 percent. The average mass of naturally occurring copper atoms is equal to the sum of the atomic mass for each isotope multiplied by its isotopic abundance. For copper, it would be (62.930 amu x 0.692) + (64.928 amu x 0.308) = 63.545 amu. The atomic weight of copper is therefore 63.545 g.

Radioactivity

About 300 combinations of protons and neutrons in nuclei are stable enough to exist in nature. Scientists can produce another 3,000 nuclei in the laboratory. These nuclei tend to be extremely unstable because they have too many protons or neutrons to stay in one piece for long. Unstable nuclei, whether naturally occurring or created in the laboratory, break apart or change into stable nuclei through a variety of processes known as radioactive decays (see Radioactivity).

Some nuclei with an excess of protons simply eject a proton. A similar process can occur in nuclei with an excess of neutrons. A more common process of decay is for a nucleus to simultaneously eject a cluster of 2 protons and 2 neutrons. This cluster is actually the nucleus of an atom of helium-4, and this decay process is called alpha decay. Before scientists identified the ejected particle as a helium-4 nucleus, they called it an alpha particle. Helium-4 nuclei are still sometimes called alpha particles.

The most common way for a nucleus to get rid of excess protons or neutrons is to convert a proton into a neutron or a neutron into a proton. This process is known as beta decay. The total electric charge before and after the decay must remain the same. Because protons are electrically charged and neutrons are not, the reaction must involve other charged particles. For example, a neutron can decay into a proton, an electron, and another particle called an electron antineutrino. The neutron has no charge, so the charge at the beginning of the reaction is zero. The proton has an electric charge of +1 and the electron has an electric charge of –1. The antineutrino is a tiny particle with no electric charge. The electric charges of the proton and electron cancel each other, leaving a net charge of zero. The electron is the most easily detected product of this type of beta decay, and scientists called these products beta particles before they identified them as electrons.

Beta decay also results when a proton changes to a neutron. The end result of this decay must have a charge of +1 to balance the charge of the initial proton. The proton changes into a neutron, an anti-electron (also called a positron), and an electron neutrino. A positron is identical to an electron, except the positron has an electric charge of +1. The electron neutrino is a tiny, electrically neutral particle. The difference between the antineutrino in neutron-proton beta decay and the neutrino in proton-neutron beta decay is very subtle—so subtle that scientists have yet to prove that a difference actually exists.

While scientists often create unstable nuclei in the laboratory, several radioactive isotopes also occur naturally. These atoms decay more slowly than most of the radioactive isotopes created in laboratories. If they decayed too rapidly, they wouldn’t stay around long enough for scientists to find them. The heavy radioactive isotopes found on Earth formed in the interiors of stars more than 5 billion years ago. They were part of the cloud of gas and dust that formed our solar system and, as such, are reminders of the origin of Earth and the other planets. In addition, the decay of radioactive material provides much of the energy that heats Earth’s core.

The most common naturally occurring radioactive isotopes are potassium-40 (see Potassium), thorium-232 (see Thorium), and uranium-238 (see Uranium). Atoms of these isotopes last, on average, for billions of years before undergoing alpha or beta decay. The steady decay of these isotopes and other, more stable atoms allows scientists to determine the age of minerals in which these isotopes occur. Scientists begin by estimating the amount of isotope that was present when the mineral formed, then measure how much has decayed. Knowing the rate at which the isotope decays, they can determine how much time has passed. This process, known as radioactive dating (see Dating Methods), allows scientists to measure the age of Earth. The currently accepted value for Earth’s age is about 4.5 billion years. Scientists have also examined rocks from the Moon and other objects in the solar system and have found that they have similar ages.

Source: Microsoft ® Encarta ® 2009

Atom

2010-08-18T01:02:00.000-07:00

Atom, tiny basic building block of matter. All the material on Earth is composed of various combinations of atoms. Atoms are the smallest particles of a chemical element that still exhibit all the chemical properties unique to that element. A row of 100 million atoms would be only about a centimeter long. See also Chemical Element.

Understanding atoms is key to understanding the physical world. More than 100 different elements exist in nature, each with its own unique atomic makeup. The atoms of these elements react with one another and combine in different ways to form a virtually unlimited number of chemical compounds. When two or more atoms combine, they form a molecule. For example, two atoms of the element hydrogen (abbreviated H) combine with one atom of the element oxygen (O) to form a molecule of water (H₂0).

Since all matter—from its formation in the early universe to present-day biological systems—consists of atoms, understanding their structure and properties plays a vital role in physics, chemistry, and medicine. In fact, knowledge of atoms is essential to the modern scientific understanding of the complex systems that govern the physical and biological worlds. Atoms and the compounds they form play a part in almost all processes that occur on Earth and in space. All organisms rely on a set of chemical compounds and chemical reactions to digest food, transport energy, and reproduce. Stars such as the Sun rely on reactions in atomic nuclei to produce energy. Scientists duplicate these reactions in laboratories on Earth and study them to learn about processes that occur throughout the universe.

Throughout history, people have sought to explain the world in terms of its most basic parts. Ancient Greek philosophers conceived of the idea of the atom, which they defined as the smallest possible piece of a substance. The word atom comes from the Greek word meaning “not divisible.” The ancient Greeks also believed this fundamental particle was indestructible. Scientists have since learned that atoms are not indivisible but made of smaller particles, and atoms of different elements contain different numbers of each type of these smaller particles.

THE STRUCTURE OF THE ATOM

Atoms are made of smaller particles, called electrons, protons, and neutrons. An atom consists of a cloud of electrons surrounding a small, dense nucleus of protons and neutrons. Electrons and protons have a property called electric charge, which affects the way they interact with each other and with other electrically charged particles. Electrons carry a negative electric charge, while protons have a positive electric charge. The negative charge is the opposite of the positive charge, and, like the opposite poles of a magnet, these opposite electric charges attract one another. Conversely, like charges (negative and negative, or positive and positive) repel one another. The attraction between an atom’s electrons and its protons holds the atom together. Normally, an atom is electrically neutral, which means that the negative charge of its electrons is exactly equaled by the positive charge of its protons.

The nucleus contains nearly all of the mass of the atom, but it occupies only a tiny fraction of the space inside the atom. The diameter of a typical nucleus is only about 1 × 10^-14 m (4 × 10^-13 in), or about 1/100,000 of the diameter of the entire atom. The electron cloud makes up the rest of the atom’s overall size. If an atom were magnified until it was as large as a football stadium, the nucleus would be about the size of a grape.

Electrons

Electrons are tiny, negatively charged particles that form a cloud around the nucleus of an atom. Each electron carries a single fundamental unit of negative electric charge, or –1.

The electron is one of the lightest particles with a known mass. A droplet of water weighs about a billion, billion, billion times more than an electron. Physicists believe that electrons are one of the fundamental particles of physics, which means they cannot be split into anything smaller. Physicists also believe that electrons do not have any real size, but are instead true points in space—that is, an electron has a radius of zero.

Electrons act differently than everyday objects because electrons can behave as both particles and waves. Actually, all objects have this property, but the wavelike behavior of larger objects, such as sand, marbles, or even people, is too small to measure. In very small particles wave behavior is measurable and important. Electrons travel around the nucleus of an atom, but because they behave like waves, they do not follow a specific path like a planet orbiting the Sun does. Instead they form regions of negative electric charge around the nucleus. These regions are called orbitals, and they correspond to the space in which the electron is most likely to be found. As we will discuss later, orbitals have different sizes and shapes, depending on the energy of the electrons occupying them.

Protons carry a positive charge of +1, exactly the opposite electric charge as electrons. The number of protons in the nucleus determines the total quantity of positive charge in the atom. In an electrically neutral atom, the number of the protons and the number of electrons are equal, so that the positive and negative charges balance out to zero. The proton is very small, but it is fairly massive compared to the other particles that make up matter. A proton’s mass is about 1,840 times the mass of an electron.

Neutrons are about the same size as protons but their mass is slightly greater. Without neutrons present, the repulsion among the positively charged protons would cause the nucleus to fly apart. Consider the element helium, which has two protons in its nucleus. If the nucleus did not contain neutrons as well, it would be unstable because of the electrical repulsion between the protons. (The process by which neutrons hold the nucleus together is explained below in the Strong Force section of this article.) A helium nucleus needs either one or two neutrons to be stable. Most atoms are stable and exist for a long period of time, but some atoms are unstable and spontaneously break apart and change, or decay, into other atoms.

Unlike electrons, which are fundamental particles, protons and neutrons are made up of other, smaller particles called quarks. Physicists know of six different quarks. Neutrons and protons are made up of up quarks and down quarks—two of the six different kinds of quarks. The fanciful names of quarks have nothing to do with their properties; the names are simply labels to distinguish one quark from another.

Quarks are unique among all elementary particles in that they have electric charges that are fractions of the fundamental charge. All other particles have electric charges of zero or of whole multiples of the fundamental charge. Up quarks have electric charges of +’. Down quarks have charges of -€. A proton is made up of two up quarks and a down quark, so its electric charge is ’ + ’ - €, for a total charge of +1. A neutron is made up of an up quark and two down quarks, so its electric charge is ’ - € - €, for a net charge of zero. Physicists believe that quarks are true fundamental particles, so they have no internal structure and cannot be split into something smaller.

Source: Microsoft ® Encarta ® 2009

Electron cloud

2009-06-06T01:37:00.001-07:00

An electron cloud representing the ground state of hydrogen, with a plus and a minus sign superimposed. The plus sign represents the location of the nucleus; the minus sign represents a possible location of the electron.

Electron cloud is a term used, if not originally coined, by the Nobel Prize laureate and acclaimed educator Richard Feynman in The Feynman Lectures on Physics (Feynman2006 Vol 1 lect 6 pg 11) for discussing "exactly what is an electron?". In the electron cloud analogy, an electron is described as a cloud surrounding the nucleus of an atom (or the nuclei of the atoms in a molecule). The thicker the cloud is in a region, the more likely that the electron, when its position is measured, will be found there. Electron clouds allows one to visualize the random nature of the position of quantum particles, as opposed to classical particles which can be located at one point.

To go into more detail, this intuitive model provides a way of visualizing an electron as a solution of the Schrödinger equation. Solutions of Schrödinger's equation are called wavefunctions, and when one takes the square of the absolute value of the wavefunction, one obtains the probability density of the position of an electron. A three-dimensional plot of the probability density, where the opacity of the cloud is proportional to the probability density, gives us the electron cloud image.

The electron cloud model evolved from the earlier Bohr model, which likened an electron surrounding an atomic nucleus to a planet orbiting the sun. From the Bohr model we get the term orbitals. The electron cloud model better describes many observed phenomena, including the double slit experiment, the periodic table and chemical bonding, and atomic interactions with light. This model demonstrates the wave nature of an electron, in that electron behavior is described as a delocalized wavelike object.

Experimental evidence suggests that the probability density is not just a theoretical model for the uncertainty in the location of the electron, but rather that it reflects the actual state of the electron.^{[citation needed]} This carries an enormous philosophical implication, indicating that point-like particles do not actually exist, and that the universe's evolution may be fundamentally uncertain. The fundamental source of quantum uncertainty is an unsolved problem in physics. Stochastic Electrodynamics is one relatively recent attempt to shed light on this question, and has met with only partial success so far.

source: wikipedia

Origin and current state I Nucleosynthesis I Earth I Rare and theoretical forms

2009-06-06T01:31:00.000-07:00

Origin and current state

Atoms form about 4% of the total energy density of the observable universe, with an average density of about 0.25 atoms/m³.^[104] Within a galaxy such as the Milky Way, atoms have a much higher concentration, with the density of matter in the interstellar medium (ISM) ranging from 10⁵ to 10⁹ atoms/m³.^[105] The Sun is believed to be inside the Local Bubble, a region of highly ionized gas, so the density in the solar neighborhood is only about 10³ atoms/m³.^[106] Stars form from dense clouds in the ISM, and the evolutionary processes of stars result in the steady enrichment of the ISM with elements more massive than hydrogen and helium. Up to 95% of the Milky Way's atoms are concentrated inside stars and the total mass of atoms forms about 10% of the mass of the galaxy.^[107] (The remainder of the mass is an unknown dark matter.)

Nucleosynthesis

Stable protons and electrons appeared one second after the Big Bang. During the following three minutes, Big Bang nucleosynthesis produced most of the helium, lithium, and deuterium in the universe, and perhaps some of the beryllium and boron.^[109]^[110]^[111] The first atoms (complete with bound electrons) were theoretically created 380,000 years after the Big Bang—an epoch called recombination, when the expanding universe cooled enough to allow electrons to become attached to nuclei.^[112] Since then, atomic nuclei have been combined in stars through the process of nuclear fusion to produce elements up to iron.

Isotopes such as lithium-6 are generated in space through cosmic ray spallation.^[114] This occurs when a high-energy proton strikes an atomic nucleus, causing large numbers of nucleons to be ejected. Elements heavier than iron were produced in supernovae through the r-process and in AGB stars through the s-process, both of which involve the capture of neutrons by atomic nuclei.^[115] Elements such as lead formed largely through the radioactive decay of heavier elements.

Earth

Most of the atoms that make up the Earth and its inhabitants were present in their current form in the nebula that collapsed out of a molecular cloud to form the Solar System. The rest are the result of radioactive decay, and their relative proportion can be used to determine the age of the Earth through radiometric dating.^[117]^[118] Most of the helium in the crust of the Earth (about 99% of the helium from gas wells, as shown by its lower abundance of helium-3) is a product of alpha decay.^[119]

There are a few trace atoms on Earth that were not present at the beginning (i.e., not "primordial"), nor are results of radioactive decay. Carbon-14 is continuously generated by cosmic rays in the atmosphere.^[120] Some atoms on Earth have been artificially generated either deliberately or as by-products of nuclear reactors or explosions.^[121]^[122] Of the transuranic elements—those with atomic numbers greater than 92—only plutonium and neptunium occur naturally on Earth.^[123]^[124] Transuranic elements have radioactive lifetimes shorter than the current age of the Earth^[125] and thus identifiable quantities of these elements have long since decayed, with the exception of traces of plutonium-244 possibly deposited by cosmic dust.^[117] Natural deposits of plutonium and neptunium are produced by neutron capture in uranium ore.^[126]

The Earth contains approximately 1.33 × 10⁵⁰ atoms.^[127] In the planet's atmosphere, small numbers of independent atoms of noble gases exist, such as argon and neon. The remaining 99% of the atmosphere is bound in the form of molecules, including carbon dioxide and diatomic oxygen and nitrogen. At the surface of the Earth, atoms combine to form various compounds, including water, salt, silicates and oxides. Atoms can also combine to create materials that do not consist of discrete molecules, including crystals and liquid or solid metals.^[128]^[129] This atomic matter forms networked arrangements that lack the particular type of small-scale interrupted order associated with molecular matter.^[130]

Rare and theoretical forms

While isotopes with atomic numbers higher than lead (82) are known to be radioactive, an "island of stability" has been proposed for some elements with atomic numbers above 103. These superheavy elements may have a nucleus that is relatively stable against radioactive decay.^[131] The most likely candidate for a stable superheavy atom, unbihexium, has 126 protons and 184 neutrons.^[132]

Each particle of matter has a corresponding antimatter particle with the opposite electrical charge. Thus, the positron is a positively charged antielectron and the antiproton is a negatively charged equivalent of a proton. When a matter and corresponding antimatter particle meet, they annihilate each other. Because of this, along with an imbalance between the number of matter and antimatter particles, the latter are rare in the universe. (The first causes of this imbalance is not yet fully understood, although the baryogenesis theories may offer an explanation.) As a result, no antimatter atoms have been discovered in nature.^[133]^[134] However, in 1996, antihydrogen, the antimatter counterpart of hydrogen, was synthesized at the CERN laboratory in Geneva.^[135]^[136]

Other exotic atoms have been created by replacing one of the protons, neutrons or electrons with other particles that have the same charge. For example, an electron can be replaced by a more massive muon, forming a muonic atom. These types of atoms can be used to test the fundamental predictions of physics.

source: wikipedia

States I Identification

2009-06-06T01:28:00.000-07:00

States

Snapshots illustrating the formation of a Bose–Einstein condensate.

Quantities of atoms are found in different states of matter that depend on the physical conditions, such as temperature and pressure. By varying the conditions, materials can transition between solids, liquids, gases and plasmas.^[93] Within a state, a material can also exist in different phases. An example of this is solid carbon, which can exist as graphite or diamond.

At temperatures close to absolute zero, atoms can form a Bose–Einstein condensate, at which point quantum mechanical effects, which are normally only observed at the atomic scale, become apparent on a macroscopic scale.^[95]^[96] This super-cooled collection of atoms then behaves as a single super atom, which may allow fundamental checks of quantum mechanical behavior.^[97]

Identification

Scanning tunneling microscope image showing the individual atoms making up this gold (100) surface. Reconstruction causes the surface atoms to deviate from the bulk crystal structure and arrange in columns several atoms wide with pits between them.

The scanning tunneling microscope is a device for viewing surfaces at the atomic level. It uses the quantum tunneling phenomenon, which allows particles to pass through a barrier that would normally be insurmountable. Electrons tunnel through the vacuum between two planar metal electrodes, on each of which is an adsorbed atom, providing a tunneling-current density that can be measured. Scanning one atom (taken as the tip) as it moves past the other (the sample) permits plotting of tip displacement versus lateral separation for a constant current. The calculation shows the extent to which scanning-tunneling-microscope images of an individual atom are visible. It confirms that for low bias, the microscope images the space-averaged dimensions of the electron orbitals across closely packed energy levels—the Fermi level local density of states.^[98]^[99]

An atom can be ionized by removing one of its electrons. The electric charge causes the trajectory of an atom to bend when it passes through a magnetic field. The radius by which the trajectory of a moving ion is turned by the magnetic field is determined by the mass of the atom. The mass spectrometer uses this principle to measure the mass-to-charge ratio of ions. If a sample contains multiple isotopes, the mass spectrometer can determine the proportion of each isotope in the sample by measuring the intensity of the different beams of ions. Techniques to vaporize atoms include inductively coupled plasma atomic emission spectroscopy and inductively coupled plasma mass spectrometry, both of which use a plasma to vaporize samples for analysis.^[100]

A more area-selective method is electron energy loss spectroscopy, which measures the energy loss of an electron beam within a transmission electron microscope when it interacts with a portion of a sample. The atom-probe tomograph has sub-nanometer resolution in 3-D and can chemically identify individual atoms using time-of-flight mass spectrometry.^[101]

Spectra of excited states can be used to analyze the atomic composition of distant stars. Specific light wavelengths contained in the observed light from stars can be separated out and related to the quantized transitions in free gas atoms. These colors can be replicated using a gas-discharge lamp containing the same element.^[102] Helium was discovered in this way in the spectrum of the Sun 23 years before it was found on Earth.

source: wikipedia

Radioactive decay I Magnetic moment I Energy levels

2009-06-06T01:25:00.000-07:00

This diagram shows the half-life (T_½) in seconds of various isotopes with Z protons and N neutrons.

Every element has one or more isotopes that have unstable nuclei that are subject to radioactive decay, causing the nucleus to emit particles or electromagnetic radiation. Radioactivity can occur when the radius of a nucleus is large compared with the radius of the strong force, which only acts over distances on the order of 1 fm.^[73]

The most common forms of radioactive decay are:

Alpha decay is caused when the nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons. The result of the emission is a new element with a lower atomic number.
Beta decay is regulated by the weak force, and results from a transformation of a neutron into a proton, or a proton into a neutron. The first is accompanied by the emission of an electron and an antineutrino, while the second causes the emission of a positron and a neutrino. The electron or positron emissions are called beta particles. Beta decay either increases or decreases the atomic number of the nucleus by one.
Gamma decay results from a change in the energy level of the nucleus to a lower state, resulting in the emission of electromagnetic radiation. This can occur following the emission of an alpha or a beta particle from radioactive decay.

Other more rare types of radioactive decay include ejection of neutrons or protons or clusters of nucleons from a nucleus, or more than one beta particle, or result (through internal conversion) in production of high-speed electrons which are not beta rays, and high-energy photons which are not gamma rays.

Each radioactive isotope has a characteristic decay time period—the half-life—that is determined by the amount of time needed for half of a sample to decay. This is an exponential decay process that steadily decreases the proportion of the remaining isotope by 50% every half life. Hence after two half-lives have passed only 25% of the isotope will be present, and so forth.^[73]

Magnetic moment

Elementary particles possess an intrinsic quantum mechanical property known as spin. This is analogous to the angular momentum of an object that is spinning around its center of mass, although strictly speaking these particles are believed to be point-like and cannot be said to be rotating. Spin is measured in units of the reduced Planck constant (ħ), with electrons, protons and neutrons all having spin ½ ħ, or "spin-½". In an atom, electrons in motion around the nucleus possess orbital angular momentum in addition to their spin, while the nucleus itself possesses angular momentum due to its nuclear spin.

The magnetic field produced by an atom—its magnetic moment—is determined by these various forms of angular momentum, just as a rotating charged object classically produces a magnetic field. However, the most dominant contribution comes from spin. Due to the nature of electrons to obey the Pauli exclusion principle, in which no two electrons may be found in the same quantum state, bound electrons pair up with each other, with one member of each pair in a spin up state and the other in the opposite, spin down state. Thus these spins cancel each other out, reducing the total magnetic dipole moment to zero in some atoms with even number of electrons.^[77]

In ferromagnetic elements such as iron, an odd number of electrons leads to an unpaired electron and a net overall magnetic moment. The orbitals of neighboring atoms overlap and a lower energy state is achieved when the spins of unpaired electrons are aligned with each other, a process known as an exchange interaction. When the magnetic moments of ferromagnetic atoms are lined up, the material can produce a measurable macroscopic field. Paramagnetic materials have atoms with magnetic moments that line up in random directions when no magnetic field is present, but the magnetic moments of the individual atoms line up in the presence of a field.

The nucleus of an atom can also have a net spin. Normally these nuclei are aligned in random directions because of thermal equilibrium. However, for certain elements (such as xenon-129) it is possible to polarize a significant proportion of the nuclear spin states so that they are aligned in the same direction—a condition called hyperpolarization. This has important applications in magnetic resonance imaging.^[79]^[80]

Energy levels

When an electron is bound to an atom, it has a potential energy that is inversely proportional to its distance from the nucleus. This is measured by the amount of energy needed to unbind the electron from the atom, and is usually given in units of electronvolts (eV). In the quantum mechanical model, a bound electron can only occupy a set of states centered on the nucleus, and each state corresponds to a specific energy level. The lowest energy state of a bound electron is called the ground state, while an electron at a higher energy level is in an excited state.^[81]

In order for an electron to transition between two different states, it must absorb or emit a photon at an energy matching the difference in the potential energy of those levels. The energy of an emitted photon is proportional to its frequency, so these specific energy levels appear as distinct bands in the electromagnetic spectrum.^[82] Each element has a characteristic spectrum that can depend on the nuclear charge, subshells filled by electrons, the electromagnetic interactions between the electrons and other factors.^[83]

An example of absorption lines in a spectrum.

When a continuous spectrum of energy is passed through a gas or plasma, some of the photons are absorbed by atoms, causing electrons to change their energy level. Those excited electrons that remain bound to their atom will spontaneously emit this energy as a photon, traveling in a random direction, and so drop back to lower energy levels. Thus the atoms behave like a filter that forms a series of dark absorption bands in the energy output. (An observer viewing the atoms from a different direction, which does not include the continuous spectrum in the background, will instead see a series of emission lines from the photons emitted by the atoms.) Spectroscopic measurements of the strength and width of spectral lines allow the composition and physical properties of a substance to be determined.^[84]

Close examination of the spectral lines reveals that some display a fine structure splitting. This occurs because of spin-orbit coupling, which is an interaction between the spin and motion of the outermost electron.^[85] When an atom is in an external magnetic field, spectral lines become split into three or more components; a phenomenon called the Zeeman effect. This is caused by the interaction of the magnetic field with the magnetic moment of the atom and its electrons. Some atoms can have multiple electron configurations with the same energy level, which thus appear as a single spectral line. The interaction of the magnetic field with the atom shifts these electron configurations to slightly different energy levels, resulting in multiple spectral lines.^[86] The presence of an external electric field can cause a comparable splitting and shifting of spectral lines by modifying the electron energy levels, a phenomenon called the Stark effect.^[87]

If a bound electron is in an excited state, an interacting photon with the proper energy can cause stimulated emission of a photon with a matching energy level. For this to occur, the electron must drop to a lower energy state that has an energy difference matching the energy of the interacting photon. The emitted photon and the interacting photon will then move off in parallel and with matching phases. That is, the wave patterns of the two photons will be synchronized. This physical property is used to make lasers, which can emit a coherent beam of light energy in a narrow frequency band.

source: wikipedia

Atom I Components

2009-06-06T01:16:00.000-07:00

Helium atom

An illustration of the helium atom, depicting the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) is in reality spherically symmetric, although for more complicated nuclei this is not always the case. The black bar is one ångström, equal to 10⁻¹⁰ m or 100,000 fm.

Classification

Smallest recognized division of a chemical element

Properties

Mass range:	1.67 × 10⁻²⁷ to 4.52 × 10⁻²⁵ kg
Electric charge:	zero (neutral), or ion charge
Diameter range:	62 pm (He) to 520 pm (Cs) (data page)
Components:	Electrons and a compact nucleus of protons and neutrons

The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element.

The name atom comes from the Greek ἄτομος/átomos, α-τεμνω, which means uncuttable, something that cannot be divided further. The concept of an atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was not indivisible. The principles of quantum mechanics were used to successfully model the atom.^[1]^[2]

Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses. Atoms can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.9% of an atom's mass is concentrated in the nucleus,^{[note 1]} with protons and neutrons having roughly equal mass. Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus.^[3] Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.

Components

Subatomic particles

Though the word atom originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various subatomic particles. The constituent particles of an atom are the electron, the proton and the neutron. However, the hydrogen-1 atom has no neutrons and a positive hydrogen ion has no electrons.

The electron is by far the least massive of these particles at 9.11 × 10⁻³¹ kg, with a negative electrical charge and a size that is too small to be measured using available techniques.^[37] Protons have a positive charge and a mass 1,836 times that of the electron, at 1.6726 × 10⁻²⁷ kg, although this can be reduced by changes to the energy binding the proton into an atom. Neutrons have no electrical charge and have a free mass of 1,839 times the mass of electrons,^[38] or 1.6929 × 10⁻²⁷ kg. Neutrons and protons have comparable dimensions—on the order of 2.5 × 10⁻¹⁵ m—although the 'surface' of these particles is not sharply defined.^[39]

In the Standard Model of physics, both protons and neutrons are composed of elementary particles called quarks. The quark belongs to the fermion group of particles, and is one of the two basic constituents of matter—the other being the lepton, of which the electron is an example. There are six types of quarks, each having a fractional electric charge of either +2/3 or −1/3. Protons are composed of two up quarks and one down quark, while a neutron consists of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles. The quarks are held together by the strong nuclear force, which is mediated by gluons. The gluon is a member of the family of gauge bosons, which are elementary particles that mediate physical forces.

Nucleus

The binding energy needed for a nucleon to escape the nucleus, for various isotopes.

All the bound protons and neutrons in an atom make up a tiny atomic nucleus, and are collectively called nucleons. The radius of a nucleus is approximately equal to fm, where A is the total number of nucleons.^[42] This is much smaller than the radius of the atom, which is on the order of 10⁵ fm. The nucleons are bound together by a short-ranged attractive potential called the residual strong force. At distances smaller than 2.5 fm this force is much more powerful than the electrostatic force that causes positively charged protons to repel each other.^[43]

Atoms of the same element have the same number of protons, called the atomic number. Within a single element, the number of neutrons may vary, determining the isotope of that element. The total number of protons and neutrons determine the nuclide. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay.^[44]

The neutron and the proton are different types of fermions. The Pauli exclusion principle is a quantum mechanical effect that prohibits identical fermions (such as multiple protons) from occupying the same quantum physical state at the same time. Thus every proton in the nucleus must occupy a different state, with its own energy level, and the same rule applies to all of the neutrons. (This prohibition does not apply to a proton and neutron occupying the same quantum state.)^[45]

For atoms with low atomic numbers, a nucleus that has a different number of protons than neutrons can potentially drop to a lower energy state through a radioactive decay that causes the number of protons and neutrons to more closely match. As a result, atoms with roughly matching numbers of protons and neutrons are more stable against decay. However, with increasing atomic number, the mutual repulsion of the protons requires an increasing proportion of neutrons to maintain the stability of the nucleus, which modifies this trend. Thus, there are no stable nuclei with equal proton and neutron numbers above atomic number Z = 20 (calcium); and as Z increases toward the heaviest nuclei, the ratio of neutrons per proton required for stability increases to about 1.5.^[45]

Illustration of a nuclear fusion process that forms a deuterium nucleus, consisting of a proton and a neutron, from two protons. A positron (e⁺)—an antimatter electron—is emitted along with an electron neutrino.

The number of protons and neutrons in the atomic nucleus can be modified, although this can require very high energies because of the strong force. Nuclear fusion occurs when multiple atomic particles join to form a heavier nucleus, such as through the energetic collision of two nuclei. For example, at the core of the Sun protons require energies of 3–10 keV to overcome their mutual repulsion—the coulomb barrier—and fuse together into a single nucleus.^[46] Nuclear fission is the opposite process, causing a nucleus to split into two smaller nuclei—usually through radioactive decay. The nucleus can also be modified through bombardment by high energy subatomic particles or photons. If this modifies the number of protons in a nucleus, the atom changes to a different chemical element.^[47]^[48]

If the mass of the nucleus following a fusion reaction is less than the sum of the masses of the separate particles, then the difference between these two values is emitted as energy, as described by Albert Einstein's mass–energy equivalence formula, E = mc², where m is the mass loss and c is the speed of light. This deficit is the binding energy of the nucleus.^[49]

The fusion of two nuclei that have lower atomic numbers than iron and nickel is usually an exothermic process that releases more energy than is required to bring them together.^[50] It is this energy-releasing process that makes nuclear fusion in stars a self-sustaining reaction. For heavier nuclei, the total binding energy begins to decrease. That means fusion processes with nuclei that have higher atomic numbers is an endothermic process. These more massive nuclei can not undergo an energy-producing fusion reaction that can sustain the hydrostatic equilibrium of a star.^[45]

Electron cloud

A potential well, showing the minimum energy V(x) needed to reach each position x. A particle with energy E is constrained to a range of positions between x₁ and x₂.

The electrons in an atom are attracted to the protons in the nucleus by the electromagnetic force. This force binds the electrons inside an electrostatic potential well surrounding the smaller nucleus, which means that an external source of energy is needed in order for the electron to escape. The closer an electron is to the nucleus, the greater the attractive force. Hence electrons bound near the center of the potential well require more energy to escape than those at greater separations.

Electrons, like other particles, have properties of both a particle and a wave. The electron cloud is a region inside the potential well where each electron forms a type of three-dimensional standing wave—a wave form that does not move relative to the nucleus. This behavior is defined by an atomic orbital, a mathematical function that characterises the probability that an electron will appear to be at a particular location when its position is measured.^[51] Only a discrete (or quantized) set of these orbitals exist around the nucleus, as other possible wave patterns will rapidly decay into a more stable form.^[52] Orbitals can have one or more ring or node structures, and they differ from each other in size, shape and orientation.^[53]

Wave functions of the first five atomic orbitals. The three 2p orbitals each display a single angular node that has an orientation and a minimum at the center.

Each atomic orbital corresponds to a particular energy level of the electron. The electron can change its state to a higher energy level by absorbing a photon with sufficient energy to boost it into the new quantum state. Likewise, through spontaneous emission, an electron in a higher energy state can drop to a lower energy state while radiating the excess energy as a photon. These characteristic energy values, defined by the differences in the energies of the quantum states, are responsible for atomic spectral lines.

The amount of energy needed to remove or add an electron (the electron binding energy) is far less than the binding energy of nucleons. For example, it requires only 13.6 eV to strip a ground-state electron from a hydrogen atom,^[54] compared to 2.23 Mev for splitting a deuterium nucleus.^[55] Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms that have either a deficit or a surplus of electrons are called ions. Electrons that are farthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism, atoms are able to bond into molecules and other types of chemical compounds like ionic and covalent network crystals.

source: wikipedia

The Expansion of the Universe

2009-05-13T20:22:00.000-07:00

In the 20th century, great strides were made in the field of astronomy. First, the Russian physicist Alexandre Friedmann discovered in 1922 that the universe did not have a static structure. Starting out from Einstein's theory of relativity, Friedmann calculated that even a tiny impulse might cause the universe to expand or contract. Georges Lemaître, one of the most famous astronomers of Belgium, was the first to recognise the importance of this calculation. These calculations led him to conclude that the universe had a beginning and that it was continuously expanding right from the outset. There was another very important point Lemaître raised: according to him, there should be a radiation surplus left over

The universe came into existence out of nothing with a Big Bang.
The present perfect system of the universe came about because of
the scattering of all particles and forces that were formed in great
harmony and order from the first moment of this big explosion.

from the big bang and this could be traced. Lemaître wasconfident that his explanations were true although theyinitially did not find much support in the scientific community. Meanwhile, further evidence that the universewas expanding began to pile up. At that time, observinga number of stars through his huge telescope, the Americanastronomer Edwin Hubble discovered that the starsemitted a red shifted light depending on their distances. With this discovery, which he made at the California MountWilson Observatory, Hubble challenged all scientists who put forwardand defended the steady state theory, and shook the very basis of the modelof the universe held until then.

Hubble's findings depended on the physical rule that the spectra oflight beams travelling towards the point of observation tend towards violetwhile the spectra of light beams moving away from the point of observationtend towards red. This showed that the celestial bodies observed from theCalifornian Mount Wilson Observatory were moving away from the earth.Further observation revealed that the stars and galaxies weren't just racingaway from us; they were racing away from each other as well. This movementof celestial bodies proved once more that the universe is expanding. InStephen Hawking's Universe, David Filkin relates an interesting point aboutthese developments:

…Within two years, Lemaître heard the news

he had scarcely dared hope for. Hubble had observed that the light fromgalaxies was red shifted, and, according to Doppler effect, this had tomean the universe was expanding. Now it was only a matter of time.Einstein was interested in Hubble's work anyway and resolved to visithim at the Mount Wilson Observatory. Lemaître arranged to give a lectureat the California Institute of Technology at the same time, and managedto corner Einstein and Hubble together. He argued his "primevalatom" theory carefully, step by step, suggesting that the whole universehad been created "on a day which had no yesterday." Painstakingly heworked through all the mathematics. When he had finished he couldnot believe his ears. Einstein stood up and announced that what he hadjust heard was "the most beautiful and satisfying interpretation I havelistened to" and went on to confess that creating the "cosmological constant"was "the biggest blunder" of his life.

The truth that made Einstein, who is considered one of the most importantscientists in history, jump to his feet was the fact that the universehas a beginning.

Further observations on the expansion of the universe gave way to new arguments. Starting from this point, scientists ended up with a model of a universe that became smaller as one went back in time, eventually contracting and converging at a single point, as Lemaître had argued. The conclusion to be derived from this model is that at some point in time, all matter in the universe was crushed together in a single point-mass that had "zero volume" because of its immense gravitational force. Our universe came into being as the result of the explosion of this point-mass that had zero volume and this explosion has come to be called the "Big Bang".

The Big Bang pointed to another matter. To say that something has zero volume is tantamount to saying that it is "nothing". The whole universe is created from this "nothing". Furthermore, this universe has a beginning, contrary to the view of materialism, which holds that "the universe has existed from eternity".

Big Bang with Evidence

Once the fact that the universe started to form after a great explosion was established, astrophysicists gave a further boost to their researches. According to George Gamow, if the universe was formed in a sudden, cataclysmic explosion, there ought to be a definite amount of radiation left over from that explosion which should be uniform throughout the universe.

In the years following this hypothesis, scientific findings followed one another, all confirming the Big Bang. In 1965, two researchers by the name of Arno Penzias and Robert Wilson chanced upon a form of radiation hitherto unnoticed. Called "cosmic background radiation", it was unlike anything coming from anywhere else in the universe for it was extraordinarily uniform. It was neither localised nor did it have a definite source; instead, it was distributed equally everywhere. It was soon realised that this radiation is the relic of the Big Bang, still reverberating since the first moments of that great explosion. Gamow had been spot-on, for the frequency of the radiation was nearly the same value that scientists had predicted. Penzias and Wilson were awarded the Nobel Prize for their discovery.

It took only eight minutes for George Smoot and his NASA team to confirm the levels of radiation reported by Penzias and Wilson, thanks to the COBE space satellite. The sensitive sensors on board the satellite earned a new victory for the Big Bang theory. The sensors verified the existence of the hot, dense form remaining from the first moments of the Big Bang. COBE captured evidentiary remnants of the Big Bang, and the scientific community was compelled to acknowledge it. Other evidence had to do with the relative amounts of hydrogen and helium in the universe. Calculations revealed that the proportion of hydrogen-helium gasses in the universe is in accord with theoretical calculations of what should remain after the Big Bang.

The discovery of compelling evidence caused the Big Bang theory to gain the complete approval of the scientific world. In an article in its October 1994 issue, Scientific American noted that "the Big Bang model was the only acknowledged model of the 20th century"

Confessions were forthcoming one by one from the names who had defended the "infinite universe" concept for years. Defending the steady-state theory alongside Fred Hoyle for years, Dennis Sciama described the final position they had reached after all the evidence for the Big Bang theory was revealed:

There was at that time a somewhat acrimonious debate between some of the proponents of the steady state theory and observers who were testing it and, I think, hoping to disprove it. I played a very minor part at that time because I was a supporter of the steady state theory, not in the sense that I believed that it had to be true, but in that I found it so attractive I wanted it to be true. When hostile observational evidence became to come in, Fred Hoyle took a leading part in trying to counter this evidence, and I played a small part at the side, also making suggestions as to how the hostile evidence could be answered. But as that evidence piled up, it became more and more evident that the game was up, and that one had to abandon the steady state theory.

The Formation Adventure Of The Atom

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The universe, whose vast dimension pushes the limits of the human's comprehension, unctions without fail, resting on sensitive balances and within a great order and has done so since the first moment of its formation. How this enormous universe has come into being, where it leads to and how the laws that maintain the order and balance within it work, have always been matters of interest to people in all ages, and still are. Scientists made countless researches into these subjects and produced various arguments and theories. For scientists who measured the order and design in the universe by using their reason and conscience, it has not been difficult at all to explain this perfection. This is because Allah, the Almighty, Who rules over the entire universe, created this perfect design and this is obvious and clear to all people who can think and reason. Allah proclaims this evident truth in the verses of the Qur'an:

In the creation of the heavens and the earth, and the alternation of night and day, there are Signs for people with intelligence. (Surat Al 'Imran: 190)

Those scientists who ignore the evidence of creation, however, have great difficulty in answering these never-ending questions. They do not hesitate to take recourse to demagoguery, false theories without any scientific basis, and, if forced into a corner, even deceptions to defend theories that are entirely opposed to reality. Yet, all developments that have taken place in science recently, up until the outset of the 21st century, lead us to a single fact: the universe was created from nothing by Allah, Who possesses superior might and infinite wisdom.

The Creation of the Universe

For centuries, people searched for an answer to the question of "how the universe came into being". Thousands of models of the universe have been put forward and thousands of theories have been produced throughout history. However, a review of these theories reveals that they all have at their core one of two different models. The first is the concept of an infinite universe without beginning, which no longer has any scientific basis. The second is that the universe was created from nothing, which is currently recognized by the scientific community as "the standard model".

The first model, which has proven not to be viable, defended the proposition that the universe has existed for an infinite time and will exist endlessly in its current state. This idea of an infinite universe was developed in ancient Greece, and made its way to the western world as a product of the materialistic philosophy that was revived with Renaissance. At the core of the Renaissance lay a re-examination of the works of ancient Greek thinkers. Thus, materialist philosophy and the concept of an infinite universe defended by this philosophy were taken off the dusty shelves of history by philosophical and ideological concerns and presented to people as if they were scientific facts.

Materialists like Karl Marx and Friedrich Engels vigorously embraced this idea, which prepared an apparently solid ground for their materialist ideologies, thereby playing an important role in introducing this model to the 20th century.

According to this "infinite universe" model which was popular during the first half of the 20th century, the universe had no beginning or end. The universe had not been created from nothing, nor would it ever be destroyed. According to this theory, which also laid the basis for materialist philosophy, the universe had a static structure. Yet, later scientific findings revealed that this theory is totally wrong and unscientific. The universe has not existed without beginning; it had a beginning and was created from nothing.

The idea that the universe is infinite, that is that it had no beginning, has always been the starting point of irreligiousness and ideologies that make the mistake of denying Allah. This is because in their view, if the universe had no beginning, then there was no creator either. Yet, science soon revealed with conclusive evidence that these arguments of the materialists are invalid and that the universe started with an explosion called the Big Bang. Coming into being from nothing had only one meaning: "Creation". Allah, the Almighty created the whole universe.

The renowned British astronomer Sir Fred Hoyle was among those who were disturbed by this fact. With his "steady-state" theory, Hoyle accepted that the universe was expanding and argued that the universe was infinite in scale and without beginning or end. According to this model, as the universe expanded, matter originated spontaneously and in quantities as large as required. This theory, which was based on extremely unworkable premises, and advanced by the sole concern of supporting the idea of an "infinite universe without beginning or end" was in direct opposition to the Big Bang theory, which was scientifically proven closer to a great number of observations. Hoyle and others continued to resist this but all scientific development worked against them.

INTRODUCTION

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"Why?"

Once the answer is found, this question is the key to a gate that leads one to a completely ifferent world. It is, at the same time, a slim line that separates those who know from those who don't.

In the world in which we live, mankind is caught up in a continuous search for the answers to many questions like "what?", "how?" and "in what way?", and can make but little headway in answering them. It is unlikely for man to make his way to the truth unless he asks himself the question "why?" about the extraordinary order and balance with which he interacts.

In this book, we will deal with the subject of "the atom", the groundwork of every animate and inanimate thing. After seeing what occurs and in what way it occurs with regards to the atom, we will seek the answers to the question "why?" The answer to this question will take us to the truth we pursue.

Since the first half of the 19th century, hundreds of scientists worked day and night to reveal the secrets of the atom. These studies that uncovered the form, motion, structure and other properties of the atom shattered the very grounds of classical physics that assumed matter to be an entity without any beginning or end, and laid the foundation for modern physics. They also produced many questions.

Many physicists, looking for answers to these questions, finally agreed that there is perfect order, unerring balance and conscious design in the atom, as in everything else in the universe.

This truth is revealed in the Qur'an sent down by Allah fourteen centuries ago. As made clear in the verses of the Qur'an, the whole universe works in perfect order because the earth, the sky and everything in between is created by Allah, Who has infinite power and wisdom.

It is certainly no wonder that everything created by Allah has extraordinary excellence and runs within a flawless order. What comes as a real surprise is man's unrelenting insensitivity towards the numerous miracles he encounters, sees, hears, and knows – including his own body – and his negligence about the reason "why" these extraordinary details are presented to him.

Though dwelling on a scientific subject, the purpose of "The Miracle in the Atom" is different from that of conventional scientific books. This book deals with the "atom", unique in being the building block of both animate and inanimate objects, with the questions "what?", "how?" and "in what way?", thereby opening the door to the answer of the question "why?" Once beyond this door, the superiority of the wisdom and knowledge of Allah, and His creation will be revealed for all to see:

Allah, there is no god but Him, the Living, the Self-Sustaining. He is not subject to drowsiness or sleep. Everything in the heavens and the earth belongs to Him. Who can intercede with Him except by His permission? He knows what is before them and what is behind them but they cannot grasp any of His knowledge save what He wills. His Footstool encompasses the heavens and the earth and their preservation does not tire Him. (Surat al-Baqara: 255)

How Does Base Concept About The Atom?

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Base concept about atom actually long known person. Concept among others come from ancient greek person thinking that is pioneered by democritus alive by the end of century fourth and beginning century fifth prechristian. Theoritical that proposed, a thing divisible be parts very little final indiscrete again that called atom. Atom word come from greek that is ”atomos” that mean ” indiscrete ”.

Be mentioned that this reason come from where sand item together form a coast. In the analogy, sand atom, and coast compound. This analogy then can related with explanation democritus towards atom can not be divided again: although a coast divisible into the sand items, this sand item is indiscrete. Democritus also have occasion to that atom thoroughly solid, and doesn't has internal structure. He also think must there empty space deliver atom to give space for the movement (like movement in water and air, or solid thing flexibility). In addition, democritus also explain that to explain character difference from different materials, atom is discriminated into form, mass and the size.

With the atom model, democritus can to explain that any we see to consist of building block smaller is called atom. But model democritus this is less has experimental evidence, but new year 1800 denunciated experimental evidence appears.

Atom Model John Dalton

In the year 1803, John Dalton develop first modern atom concept. Model Dalton put attention predominantly in atom chemistry character, that is how does atom form compound, than try to explain atomic physics character. principal concept from model dalton:

1. A element consists of particle very little and indiscrete again be called atom.

2. All atoms from certain element has identical characteristics, distinguish them with element atom other.

3. Atom can not be created, destroyed, or changed to be atom from element other.

4. Compound formed when does different element atoms unite one another in a certain ratio.

5. Total and atom kind constant in certain compound.

First point from theory Dalton relates to greek person explanation about atom, that is a little unit with atom other to forms larger ones compound. Dalton also can to realize about element character existence that vary explainable with atom assorted existence proof, each has characteristics that vary. third point from model dalton show that irreversible atom by chemistry. This showed with how has salt be caned take although dissolve in water. Point fourth and fifth describe to how atoms can form chemistry compound. These concepts is correctly explains compound formation manner, and still used until now. Model Dalton, for example, can explain that water be different compound (with character and characteristic differ) from hydroxide hydrogen because has 1 slimmer hydrogen atom in every the compound than that has hydroxide hydrogen. Although theory dalton last for explain atom existence, but atomic structure stills not yet explained and why does different element has character and different characteristic stills not yet answered.

Atom Model J.J. Thomson

In the early 1900 denunciated, J.J. Thomson propose new atom model followed electron particle existence and proton. Because experiment shows proton has mass far bigger is compared electron, so model Thomson describe atom as proton single big. In particle proton, Thomson put into electron that counteract positive load existence from proton. Follow Thomson, atom consists of a circle contains positive densely load merata. In this positive load is widespread electron with begative charge magnitude equal to positive load. Popular manner to describes this model with considers electron as raisin (plumb) in pudding proton, so that this model is given plum cake model name (plumb-pudding model).

Although atom model Thomson first that put into existence concept proton and electron that contain, model thomson doesn't can to pass by observation in experiments next. As note, proton that used in model thomson this not particle proton that found at model moderner. Even actually can be said model thomson can not has proton, but a cell contains positive.

Atom model influence Dalton visible clearly in model Thomson. Dalton take a gamble on that atom solid goods, and Thomson support this brainchild in the model with groups electron and proton together.

Atom Model Rutherford

In the year 1910, Ernest Rutherford do effort in this model truth with does now being known as Rutherford scattering experiment (rutherford scattering experiment). Rutherford find partikel-α, a particle that radiated by radioactive atom, in the year 1909. This particle has positive load, and the fact us now is knowing that partikel-α like helium atom is released from the electron, give it load 2+. in this scattering experiment, current partikel-α this aimed to gold foil. This gold foil be chosen by rutherford because can be made very thin--only as thick as several gold atoms. Moment partikel-α across gold foil, Rutherford can measures how many partikel-α be scatterred by gold atom with will watch light flash partikel-α struck sail scintilator. Be atomic theory thomson, rutherfod berhipotesa partikel-α be turned a little, moment proton gold averses partikel-α contain tall positive.

But practically, Rutherford scattering experiment shows result clearly averse hypothesis and of course atom model thomson. Rutherfod find a large part alpha particle can penetrated gold foil without turned. At the same time, Rutherford also find alpha particle that turned a little, but considerably surprise, Rutherford also find several alpha particles that turned in corner very sharp return to radioactive source. To explain existence a large part partikel-α penetrated gold foil without turned, Rutherford then develop atomic nucleus model. In this model, Rutherford laid a proton big (like experiment and previous model) at atom centre. Rutherford theorize that around proton found big space empty from all particle except electron rarely. This big air-g gives alpha particle existence reason is not turned. Alpha particle that turned a little estimated via enough near from proton so that turned by electrostatic style. While several alpha particles that turned to return to source is estimated experience collision with kernel so that electrostatic style bounce back has toes.

atom model niels bohr

Atom Model Niels Bohr

In the year 1913 Niels Bohr try to explain Bohr atom model passes electron concept that follow orbit surrounds atomic nucleus that contain proton and neutrons. Follow Bohr, only found orbit in number certain, and difference delivers orbit one with other orbit distance from atomic nucleus. Electron existence either in low orbit also tall thoroughly depending by electron energy stage. So that electron at low orbit has energy smaller than electron at orbit higher. Bohr connect electron that orbed and observation towards gas spectrum pass a thinking that amount of energy that contained in electron alterable, and therefore electron can change the orbit depending from the energy change. in electricity current use situation passes by lowpressure gas, electron is de-eksitasi and move to lower orbit. In this change, electron loses amount of energy that be orbit second energy level difference. Energy that radiated this visible in the form of a photon the wave long light base on orbit second energy level difference.

Shortly, Bohr propose:

1. Electron in atom moves to surround kernel in certain tracks, doesn't send out light energy. Those electron tracks is called skin or electron energy level.

2. Electron can move from one track to other track.

3. Electron transfer from tall energy level to low is espoused energy emission. electron transfer from low energy level to tall is being espoused energy absorption.

4. Electron that move in the track presents in the situation stationary, mean electron doesn't send out light or absorb energy.

Although bohr atom model last for memodel hydrogen spectrum, this model is proved insufficient to predicts element spectrum complexer.

Atom Model James Chadwick

In the year 1932, atom model Rutherford modified a little by neutrons invention existence by james chadwick. Chadwick find that shoting partikel-α towards beryllium can produce neutrons, particle not contain, but with mass a little bigger is compared mass proton. So that, contemporary atom model model with big atomic nucleus that contains proton and neutrons is surrounded by electron cirrus. Also explain why atomic mass heavier than total mass proton and the electron.

With explanation base about atom fundamental part likes electron, proton, and neutrons, so can be maked model existence more complex and complete again from atom enough can explain character and atom characteristics and atom compound.

Modern Atom Model

Modern atom model work result researchers from year 1920 denunciateds up to in this time. Atom model declare that motion less electron in certain track and correct track from electron can not. Theory in this time declare that there is region in where found electron. This region is called with electron cloud.