Oxidation Number

Oxidation number (also called oxidation state) is the imaginary charge left on an atom once every other atom in the compound is notionally "removed," each atom being assigned its usual oxidation state according to a fixed set of rules. It provides a systematic way to track electron transfer in redox reactions, including ones with no oxygen or hydrogen involved at all. The assignment rules are:

1. The oxidation state of a free (uncombined) element is zero (e.g. each atom in H₂, Cl₂, Na, S₈).
2. For a monatomic ion, the oxidation state equals the ion's net charge (e.g. Na⁺ is +1, Cl⁻ is −1).
3. The algebraic sum of the oxidation states of all atoms in a neutral molecule is zero; in a polyatomic ion, it equals the ion's net charge (e.g. in H₂SO₄: 2×(H) + (S) + 4×(O) = 0; in SO₄²⁻: (S) + 4×(O) = −2).
4. Hydrogen is +1 in almost all its compounds, except in metal hydrides (e.g. NaH), where it is −1.
5. Fluorine is −1 in every one of its compounds.
6. Oxygen is normally −2, except in peroxides (e.g. H₂O₂, where it is −1), superoxides (e.g. KO₂, where it is −½), and compounds with fluorine (e.g. OF₂, where it is +2).
7. Alkali metals are always +1 and alkaline earth metals are always +2 in their compounds. …