We use the following rules for assigning oxidation numbers:

1.For an atom in its elemental form, the oxidation number is always zero.

2.For any monatomic ion, the oxidation number equals the charge on the ion.

3.Nonmetals usually have negative oxidation numbers, although they can sometimes be positive:

3a. The oxidation number of oxygen is usually -2 in both ionic and molecular compounds.

3b. The oxidation number of hydrogen is +1 when bonded to nonmetals
and -1 when bonded to metals.

3c. The oxidation number of fluorine is -1 in all compounds. The other halogens have an oxidation number of -1 in most binary compounds.

4.The sum of the oxidation numbers of all atoms in a neutral compound is zero. The sum of the oxidation numbers in a polyatomic ion equals the charge of the ion.