The oxidation number gives the extent to which a specie is oxidized in a reaction.
This number is assigned based on some rules:
Elements in combined state whose atoms combines with themselves have an oxidation number of zero.
The charge carried on simple ions gives their oxidation number.
Algebraic sum of all the oxidation numbers of atoms in neutral compound is zero. In an ion with more than one kind of atom, the charge on it is the oxidation number.
