Oxidation numbers are assigned using a strict set of rules. The rules state that each atom in its elemental form has an oxidation number of zero. Another key rule states that ions have oxidation numbers equal to their individual charges.
Oxidation numbers are numbers that can be assigned to individual atoms or ions that are present in compounds and reactions. The numbers can be positive, negative or zero. Oxygen always has an oxidation number of either -1 or -2, and hydrogen always has an oxidation number of +1 or -1.
The remaining elements have oxidation numbers that correlate with their group numbers. For example, Group 2A elements have an oxidation number of +2, and Group 1A elements have an oxidation number of +1.