oxidation number

Meaning: An oxidation number (oxidation state) is the charge an atom would have if every bonding electron pair were assigned to the more electronegative atom. It is a bookkeeping tool for electrons. Working rules: a free element is 0; a monatomic ion equals its charge; fluorine is −1; group 1 is +1 and group 2 is +2; oxygen is usually −2 (−1 in peroxides); hydrogen is usually +1 (−1 in metal hydrides).

Equation: Σ(oxidation numbers of all atoms) = overall charge of the species, in units of the elementary charge e. Example: in MnO₄⁻, Mn + 4(−2) = −1, so Mn = +7.

On the MCAT: Assign numbers on both sides of a reaction to find which species is oxidized (number rises) and which is reduced (number falls). Then name the reducing agent (the species oxidized) and the oxidizing agent (the species reduced), or find n, the electrons transferred, for ΔG° = −nFE°. Organic questions use it for carbon, as in alcohol → aldehyde → carboxylic acid.

Common confusion: Oxidation number is not formal charge. Formal charge splits shared electrons equally, while oxidation number gives them to the more electronegative atom. It also need not be a real charge in a covalent compound. And the oxidizing agent is the species that gets reduced.