Rules for Assigning OxidationNumbers
First, oxidation numbers are always assigned on a per atom basis; second, treat the rules in order of decreasing importance—the first rule that applies takes precedence over any subsequent rules that seem to apply.
1.	The oxidation number of the atoms in any free, uncombined element is zero. This includes polyatomic elements such as H2, O2, O3, P4, and S8.
2.	The oxidation number of an element in a simple (monatomic) ion is equal to the charge on the ion.
3.	The sum of the oxidation numbers of all atoms in a compound is zero.
4.	In a polyatomic ion, the sum of the oxidation numbers of the constituent atoms is equal to the charge on the ion.
5.	Fluorine has an oxidation number of -1 in its compounds.
6.	Hydrogen has an oxidation number of +1 in compounds unless it is combined with metals, in which case it has an oxidation number of -1. Examples of these exceptions are NaH and CaH2.
7.	Oxygen usually has an oxidation number of -2 in its compounds. There are some exceptions:
a.	Oxygen has an oxidation number of -1 in hydrogen peroxide, H2O2, and in peroxides, which contain the O22- ion; examples are CaO2 and Na2O2.
b.	Oxygen has an oxidation number of - ½ in superoxides, which contain the O2- ion; examples are KO2 and RbO2.
c.	When combined with fluorine in OF2, oxygen has an oxidation number of +2.
8.	The position of the element in the periodic table helps to assign its oxidation number:
a.	Group IA elements have oxidation numbers of +1 in all of their compounds.
b.	Group IIA elements have oxidation numbers of +2 in all of their compounds.
c.	Group IIIA elements have oxidation numbers of +3 in all of their compounds, with a few rare exceptions.
d.	Group VA elements have oxidation numbers of -3 in binary compounds with metals, with H, or with NH4+. Exceptions are compounds with a Group VA element combined with an element to its right in the periodic table; in this case, their oxidation numbers can be found by using rules 3 and 4.
e.	Group VIA elements below oxygen have oxidation numbers of -2 in binary compounds with metals, with H, or with NH4+. When these elements are combined with oxygen or with a lighter halogen, their oxidation numbers can be found by using rules 3 and 4.
f.	Group VIIA elements have oxidation numbers of -1 in binary compounds with metals, with H, with NH4+, or with a heavier halogen. When these elements except fluorine (i.e., Cl, Br, I) are combined with oxygen or with a lighter halogen, their oxidation numbers can be found by using rules 3 and 4.

Assign oxidation numbers to each element in the following compounds: (a) N2O4, (b) NH3, (c) HNO3, (d) NO3-, (e) N2

(a)	N2O4 – oxygen is -2,  there are 4 O giving a sum of -8, the sum of the oxidation numbers has to be 0, so nitrogen is +4

(b)	NH3 – hydrogen is +1, there are 3 H giving a sum of +3, the sum of the oxidation numbers has to be 0, so nitrogen is -3

(c)	HNO3 – Hydrogen is +1 and Oxygen is -2, combined they give -5, the sum has to be 0, so nitrogen is +5

(d)	NO3- – Oxygen is -2, combined they give -1, so nitrogen is +5

(e)	N2 – oxidation number for any element in its standard state is 0

+4

-2

+1

-3

+1

+5

-2

+5

-2

0