The following are some rules for assigning oxidation numbers.

1. Any element in the free state (not combined with another element) has an oxidation number of zero, regardless of the complexity of the molecule in which it occurs. Each atom in Ne, O2, N2, Cl2, P4, S8, etc. has an oxidation number of zero.

2. The oxidation number of any monoatomic ion equals the charge on the ion. Ion Oxidation Number

Ca2+ +2 Fe2+ +2 Fe3+ +3 F- -1

3. The sum of oxidation numbers of all the atoms in a compound is zero.

4. The charge of a polyatomic ion is considered its "net" oxidation number. The sum of the oxidation numbers of the atoms in a compound must add up to the net oxidation number of the polyatomic ion.

5. Fluorine has an oxidation number of -1 in all its compounds (except F2)

6. Hydrogen in compounds has an oxidation number of +1 (except in metal hydrides such as NaH, LiH, KH, etc. where its oxidation number is -1).

7. Oxygen in compounds is assigned an oxidation number of -2 (except in peroxides where its oxidation number is -1, H2O2, and in OF2 where it is +2, superoxides [KO2 & RbO2]) where oxygen has an oxidation number of - 1/2).

8. Some elements exhibit only one common oxidation number.

a Group IA elements always have and oxidation number of +1 in compounds.

b. Group IIA elements always have and oxidation number of +2 in compounds.

c. Boron and aluminum always have and oxidation number of +3 in compounds.

d. Group VA elements have oxidation numbers of -3 in binary compounds with metals and H. In compounds with non-metals, the oxidation state can differ and can even be positive.

+ e. In binary compounds with metals, H, or NH4 , the elements of Group VIA (S, Se, Te) have an oxidation number of -2. In compounds with oxygen or halogen, the oxidation state can differ and can even be positive.

+ f. In binary compounds with metals, H, or NH4 , the elements of Group VIIA (Cl, Br, I) have an oxidation number of -1. In compounds with oxygen or lighter halogen, the oxidation state of the halogen (except fluorine) can differ and can even be positive.

9. Some elements show more than one oxidation number, depending on the compound. The oxidation number of these elements can be determined by assigning oxidation numbers to the most electronegative element first then oxidation numbers to any elements in the compound that are contained in Group IA of IIA. Finally assign an oxidation state to the remaining element such that the sum of the oxidation states add up to the net oxidation number of the compound.

Examples:

CO2 O is -2 for each atom; 2( -2) = -4 C must be +4 Check +4+2(-2) = 0

- NO3 O is -2 for each atom; 3(-2) = -6 N must be +5 Check +5+3(-2) = -1

S2O32- O is -2 for each atom; 3(-2) = -6 S must be +2 Check 2(+2) + 3(-2) = -2

CH4 H is +1 for each atom; 4(+1) = +4 C must be -4 Check 4(+1) + (-4) = 0