Atomic Properties

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