1. Oxidation Numbers

2. The Rules
All free uncombined elements have an oxidation number of zero (In diatomic elements like F2, each fluorine’s oxidation number is 0)
Hydrogen has an oxidation number of +1
(exception: metal hydrides oxidation number -1)
Oxygen has an oxidation number of -2
(exception: peroxide oxidation number -1, OF2 +2)

3. The Rules The alkali metals have an oxidation state of +1
The alkaline earth metals have an oxidation state of +2
The oxidation number of Aluminum (Al) is always +3
The halogens have an oxidation state of −1 (except when they are bonded to oxygen or with another halogen)

4. The Rules
In a neutral molecule, the sum of the oxidation numbers of all of the atoms must be zero. 
Ex.  In H2O, each hydrogen is +1 and the oxygen is -2.  So, (2 x +1) + (-2) = 0. 
In a polyatomic ion, the sum of oxidation numbers of all the elements in the ion must be equal the net charge of the ion. 
Ex.  In the polyatomic ion known as hydroxide (OH-), the oxygen is -2 and the hydrogen is +1.  So, (-2) + (+1) = -1, the same as the charge on the hydroxide ion (OH-)

5. Oxidation Number Examples
Give the oxidation number of Na in Na2O?
O is -2; the two Na must be +1 each
Give the oxidation number of Cl in ClO¯?
The O is -2, but since a -1 must be left over, then the Cl is +1
Give the oxidation number for each element in KMnO4?
K = +1; O = -2 Mn = +7. There are 4 oxygens for a total of -8, K is +1, so Mn = +7.

6. References Web sites for practice!!!