Question

what are rules of finding oxiadtion state

Answer 1

Rules for Assigning Oxidation Numbers to Elements. 
Rule 2: The oxidation number of a monatomic (one-atom) ion is the same as the charge on the ion, for example: 
rule 3: The sum of all oxidation numbers in a neutral compound is zero.

Answer 2

1. An atom in a pure element or molecule such as H2 has an oxidation number of zero.

2. For ions consisting of a single atom, the oxidation number is equal to the charge on the ion.

3. Fluorine is always -1.

4. Chlorine, bromine and iodine are -1 except in compounds with fluorine or oxygen (both of which have higher electronegativities).

5. The oxidation number of hydrogen is +1 except when bound to a metal (hydrogen is more electronegative).

6.  The oxidation number of oxygen is -2.

7.  The sum of the oxidation numbers in a neutral compound must be zero.

8.  The sum of the oxidation numbers for a polyatomic ion must equal the ion charge.