.....when all the bonding pairs of electrons are removed, with the charge assigned to central atom:
1. The oxidation number of a free element is always 0.
2. The oxidation number of a mono-atomic ion is equal to the charge of the ion.
3. For a given bond, X-Y, the bond is split to give X+ and Y−, where Y is more electronegative than X.
4. The oxidation number of H is +I, but it is -I in when combined with less electronegative elements.
5. The oxidation number of O in its compounds is usually -II, but it is -I in peroxides.
6. The oxidation number of a Group 1 element in a compound is +I.
7. The oxidation number of a Group 2 element in a compound is +II.
8. The oxidation number of a Group 17 element in a binary compound is -I.
9. The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.
10. The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.
The given rules are a formalism. When we take an iron salt, or an iron complex, i.e. FeSO4, or FeCl2, or Fe(OH2)2+6, the charge on the IRON ATOM is formally II+; i.e. it is a ferrous ion. In Fe2(SO4)3, or FeCl3, or Fe(OH2)3+6, we has Fe3+, the ferric ion, Fe(III+). Iron complexes with different oxidation states are more rare.
