Summary

Our knowledge of the electron configuration of atoms is based on studies of the properties of light and of the spectra of atoms. Because of the electrons' tiny mass and high energies, their properties are similar to those of light.

Each electron in a ground-state atom has unique energy properties. In describing these properties, we assume that the nucleus of an atom is sur- rounded by energy levels; the energy associated with these levels increases as their distance from the nucleus increases. Each energy level is divided into sublevels that differ slightly from one another in energy. The sublevels are divided into orbitals, each of which can contain two electrons spinning in opposite directions on their axes. The electron configuration of a particular atom is described by stating how many electrons are in each sublevel of a particular principal energy level. If electron spin is important, box diagrams are used. Core notation emphasizes the outer-level electrons. If only the valence electrons are of interest, Lewis structures are used.

Although the periodicity of elements was clearly recognized in 1869 when the periodic table was first proposed, it is only in this century that the relation- ship of an element's electron configuration to its location in the periodic table has been clearly defined. Elements in the same long columns of the table have the same valence-shell configuration and have very similar chemical properties.

Elements in the same row of the table show a reasonably regular progression of properties, particularly with respect to atomic radius, metallic nature, and ionization potential. Elements lose or gain electrons to become ions. Metals become cations by losing electrons, thus gaining a positive charge. Nonmetals become anions by gaining electrons, thus attaining a negative charge. Polyatomic ions are groups of atoms with an excess or deficiency of electrons.

The oxides of metals react with water to yield hydroxide ions; hydroxides are a subset of a larger group called bases. Oxides of nonmetals typically react with water to form acids. The solution of an acid contains an excess of hydrogen ions; that of a base, an excess of hydroxide ions.