How to find oxidation number on redox reactions

* The oxidation number of an atom in an elementary substance is 0. This means that the oxidation number of an O atom in O2 is 0.

* The oxidation number of a Group IA atom in any compund is +1; The oxidation number of a Group IIA atom in any compound is +2.
* The oxidation number of fluorine is -1 in all of its compounds.
* The oxidation number of chlorine, bromine, and iodine is -1 in any compound containing only two elements.
* The usual oxidation number of oxygen in a compound is -2. The major exceptions of this rule are peroxides, like H2O2, which have an oxidation number of -1.
* The oxidation number of hydrogen in most compounds is +1.
* The sum of the oxidation numbers in a compound is always zero. For something that is an ion consisting of two atoms (a polyatomic ion), the oxidation numbers add up to the charge on the ion.