VALENCES

Valence is a measure of the combining capacity of an atom, ion, or radical. It describes how many bonds an element tends to form or how many electrons it can gain, lose, or share to achieve a stable electronic configuration. The concept originated in early chemistry to account for how elements combine with hydrogen and with each other.

In modern chemistry, valence is discussed in two related senses: valence as the number of bonding electrons available for forming bonds, and valence as the overall capacity to form bonds or carry charge in a compound. For main-group elements, valence roughly corresponds to the group number: carbon typically forms four bonds (valence 4), nitrogen three, oxygen two, hydrogen one, and the halogens one. Transition metals complicate the picture, with multiple possible oxidation states and changing valence depending on ligands and environment.

Valence can be defined in different ways. Covalent valence is the number of covalent bonds an atom tends to form; ionic valence is related to the magnitude of its ionic charge in a compound. In historical chemistry, primary valence referred to oxidation state, while secondary valence described coordination number, the number of atoms or groups attached to a central atom in a molecule or complex. Today, oxidation state is commonly used for electronic bookkeeping, while valence remains a useful, though more historical, descriptor in teaching and in coordination chemistry.

Examples include carbon with valence 4; oxygen with valence 2; nitrogen typically 3 or, in some compounds, 5; sulfur with valence 2, 4, or 6; and hydrogen with valence 1. In coordination compounds, a metal’s valence often corresponds to its coordination number, the number of ligands attached.