Oxidation number is the apparent charge of an atom in a compound.

Rules for assigning oxidation numbers.

Follow my order when you assign the numbers and you never can go wrong.

1. Free elements are assigned an oxidation state of zero.

ex. Fe, Ag, Br₂

2. The Group 1 metals (Li, Na, K, Rb, and Cs) in compounds are always assigned an oxidation state of +1.

ex NaCl  (Na  +1, Cl -1)

3. The Group 2 metals (Be, Mg, Ca, Sr, Ba, and Ra) and also Zn and Cd in compounds are always assigned an oxidation state of +2.

4.Fluorine in compounds is always assigned an oxidation state of -1. (The most electronegative element.)

5.  Hydrogen in compounds is assigned an oxidation state of +1.  

Exceptions Metal hydrides  (you already assigned group 1, so H must be -1)

NaH   (Na +1,  H-1)

6. Oxygen in compounds is assigned an oxidation state of -2.

Exceptions Peroxides  (you already assigned hydrogen +1, so O must be -1)

H₂O₂  (H +1,  O-1)

With Fluorine (you already assigned fluorine -1, so O must be +2)

OF₂ ( O+2, F-1)

7. The sum of the oxidation states of all that atoms in a species must be equal to the net charge on the species.

SO₄^2-

Since each O is -2 and the ox. numbers add to 2-

 S + 4(-2)=-2

S is +6

8. If all else fails, halogen in compounds is assigned an oxidation state of -1.

Past Regents Questions involving Oxidation Numbers

Jan 2007-45 What is the oxidation number of chromium in the chromate ion, CrO4^2–?
(1) +6   (3) +3
(2) +2   (4) +8

Jan 2006-21 What is the oxidation number assigned to manganese in KMnO4?
(1) +7     (3) +3
(2) +2     (4) +4

June 2009-44 Given the balanced equation representing a reaction:
Fe₂O₃ + 2Al--> Al₂O₃ + 2Fe
During this reaction, the oxidation number of Fe changes from
(1) +2 to 0 as electrons are transferred
(2) +2 to 0 as protons are transferred
(3) +3 to 0 as electrons are transferred
(4) +3 to 0 as protons are transferred

Jan 2009-44 What is the oxidation number of sulfur in Na₂S₂O₃?
(1) -1      (3) +6
(2) +2     (4) +4