Rules to calculate the oxidation number
1.The oxidation number of an atom in pure elemental form is always zero ( H2, Cl2, Zn, N2, S8, P4 )
2.The oxidation number of fluorine is always –1.
3.The oxidation number of alkali metals is always +1 and that of alkaline earth metals is +2
4.The oxidation number of oxygen in most of the compounds is –2. But in peroxides and superoxides the oxidation state is –1 and –½ respectively
5.The oxidation number of hydrogen in most of the compounds is +1. But in metal hydrides like NaH, CaH2 the oxidation number is –1.
6.In a molecule the sum of oxidation numbers of all the elements is zero.
7.In a polyatomic ion, the sum of the oxidation numbers of all the atoms is equal to the charge on the ion.