When elements combine to produce a compound, each element is assigned an “apparent” charge. This apparent charge, the charge an atom would have if both electrons in each bond were assigned to the more electronegative element, may be positive or negative. It is called the oxidation number or state of the element in the compound.
Oxidation numbers are very useful in keeping track of what happens to electrons when various elements combine to form compounds. By remembering a few generalizations concerning oxidation numbers, the correct chemical formulas for a large number of compounds can be written and it becomes unnecessary to memorize chemical formulas. The following rules will help to assign oxidation states:
- Any element in its free state, meaning not combined with another element has an oxidation number of zero. Eg: Pb, C, O2, Ne has an oxidation number of zero.
- The oxidation number of any monoatomic ion is equal to the charge on the ion. Here are some examples: Na2+ has a charge of +2, Fe3+ has a charge of +3, and Cl– has a charge of -1
- Oxygen is assigned an oxidation number of -2, except in peroxides when it is -1. The oxidation number of oxygen is -2 in MgO, BaO, Fe2O3, and K2O
- Hydrogen has an oxidation number of +1, except in metal hydrides such as FH, when it is -1. Examples of +1 charge are H2O, H2SO4 and HNO3.
- Some elements exhibit only one oxidation states in certain types of compounds:
a) The elements of Group IA always have an oxidation number of +1 in compounds.
b) The elements of Group IIA always have an oxidation number of +2 in compounds.
c) Boron and aluminum always possess an oxidation number of +3 in compounds.
d) The nonmetallic elements of VIA (O, S, Se and Te.