.....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. FeSO_4, or FeCl_2, or Fe(OH_2)_6^(2+), the charge on the IRON ATOM is formally II+; i.e. it is a "ferrous ion". In Fe_2(SO_4)_3, or FeCl_3, or Fe(OH_2)_6^(3+), we has Fe^(3+), "the ferric ion", Fe(III+). Iron complexes with different oxidation states are more rare.