Rules to work out oxidation states:
An atom in its elemental state has an oxidation number of 0.
Na, H2, Cl2, S, Xe
2. An atom in a monoatomic ion has an oxidation number identical to its charge.
Na+ , Ba+2, Al3+ ,Br– , S2–
The oxidation number is equal to the charge on the monoatomic ion.
3. An atom in a polyatomic ion or a molecular compound usually has the same oxidation number it would have if it were in a monoatomic ion.
a. Elements to the left on the periodic table are “cation like” and have positive oxidation numbers
b. Elements to the right on the periodic table are “anion like” and have negative oxidation numbers.
c. Oxygen often has a –2 oxidation number, but can have a –1 oxidation number in the peroxide ion,
H2O (O –2 oxidation number)
HOOH (O –1 oxidation number)
d. Halogens usually have an oxidation number of –1,