Here are some rules:
1. The oxidation state of a free element (uncombined element) is zero.2. For a simple (monatomic) ion, the oxidation state is equal to the net charge on the ion.3. Hydrogen has an oxidation state of +1 and oxygen has an oxidation state of −2 when they are present in most compounds.
The oxidation number of Fe in Fe(NO3)2 is +3.