D3.5 Valence Electrons

Valence, the combining power of an atom, was defined earlier. Ground-state electron configurations of atoms provide insights into valence. For example, why does sodium oxide have the formula Na₂O but magnesium oxide is MgO? When two atoms approach and form a chemical bond, the electron density farthest from the nucleus of each atom, in the higher-energy orbitals, interact with electron density in the other atom. In comparison, electrons in lower-energy orbitals, whose electron density is closer the nucleus, do not directly interact with the other atom.

Hence, electrons can be separated into two groups: valence electrons occupy the outermost orbitals of an atom; core electrons occupy inner orbitals, with electron density closer to the nucleus. When an electron configuration is written using the noble-gas notation, all electrons represented by the noble-gas symbol in brackets are core electrons. Electrons beyond the noble-gas configuration are valence electrons if they are in the outermost shell of the atom (have the highest n value) or if they are in incompletely filled subshells.

For example, consider vanadium, V: [Ar]4s²3d³. There are five valence electrons: two 4s electrons and three 3d electrons. There are 18 core electrons in the 1s, 2s, 2p, 3s, and 3p subshells. The fact that vanadium has five valence electrons results in vanadium forming compounds in which the valence of vanadium ranges from 2 to 5. For example, fluorides of vanadium have formulas VF₂, VF₃, VF₄, and VF₅.

An American chemist, G. N. Lewis, suggested a simple way to keep track of the number of valence electrons: draw dots around the symbol of an element to represent the valence electrons. The element symbol then represents the nucleus and core electrons of an atom. A diagram in which dots represent valence electrons is called a Lewis diagram. Lewis diagrams are most useful for the main-group (representative) elements. Here are Lewis diagrams for atoms of elements in the third row of the periodic table:

When drawing a Lewis diagram, dots are added one at a time to each of the four sides of the element symbol. If there are more than four dots to add, dots are paired. Lewis originated the idea that when an atom bonds to another atom, the valence electrons rearrange to form an octet, a stable configuration of valence electrons (s²p⁶) that corresponds to a noble gas. Thus electron configurations and Lewis diagrams for atoms can predict how an atom forms chemical bonds, an idea that we will explore later.