Again the problem is to balance the charge. In order to name compounds we frequently need this information which is obtained from oxidation numbers. Oxidation numbers are in essence an electronic accounting method in which electrons are assigned to a particular atom in a bond or interaction. As such they give an approximate picture of where electrons actually reside in compounds. We will find this information very useful later when we look at particular types of chemical reactions.
Oxidation numbers are essential for nomenclature.
For monoatomic ions the oxidation number equals the charge. For a compound the sum of the oxidation numbers of the atoms equals 0. For a polyatomic ion the sum of the oxidations numbers of the atoms equals the charge on the ion.