Rules for Assigning OxidationNumbers
1.	The oxidation number of the atoms in their elemental state is zero. This includes polyatomic elements such as H2, O2, O3, P4, and S8.
2.	The oxidation number of an atom in a monatomic ion is equal to the charge on the ion.
3.	The sum of the oxidation numbers of the atoms in a neutral compound is zero.
4.	In a polyatomic ion, the sum of the oxidation numbers of the atoms is equal to the charge on the ion.
5.	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. 
6.	Oxygen usually has an oxidation number of -2 in its compounds. There are some exceptions:
a.	Oxygen has an oxidation number of -1 in peroxide
b.	Oxygen has an oxidation number of - ½ in superoxides.
c.	When combined with fluorine in OF2, oxygen has an oxidation number of +2.
7.	The position of the element in the periodic table helps to assign its oxidation number:
a.	Group I elements have oxidation numbers of +1.
b.	Group II elements have oxidation numbers of +2.
c.	Group III elements have oxidation numbers of +3.
d.	Group V elements have oxidation numbers of -3 in binary compounds with metals.
e.	Group VI elements below oxygen have oxidation numbers of -2 in binary compounds with metals, with H, or with NH4+. 
f.	Group VII elements have oxidation numbers of -1 in binary compounds with metals, with H, with NH4+, or with a heavier halogen. 

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