Valence and Oxidation State: A Comparison of Their Value and Limitations as Simple Chemical Models

Valence and oxidation state are two longstanding models that each remain effective ways of organizing and rationalizing the chemistry of the elements. Broadly, the valence or valence number (VN) of an atom in a compound is defined here as the number of its valence electrons involved in bonding to other atoms, whereas the oxidation state (OS) is defined as the charge remaining on an atom after ionic approximation according to electronegativity differences associated with its heteronuclear bonds. While these definitions frequently lead to the values of VN and OS being numerically equivalent, this Tutorial focuses more on those circumstances where they are not equal. Examples of compounds where, for at least some atoms, VN ≠ OS include Lewis acid–base adducts, complexes with negative oxidation states, complexes with coordinated cationic ligands, and species with homonuclear element–element bonds. The merits and shortcomings of both concepts as approximate models are critically discussed, and a series of recommendations are made on how they should be used in practice.