You assign oxidation numbers to the elements in a compound by using the Rules for Oxidation Numbers.

Explanation:

The oxidation number of a free element is always 0.

The oxidation number of a monatomic ion equals the charge of the ion.

The oxidation number of H is +1, but it is -1 in when combined with less electronegative elements.

The oxidation number of O in compounds is usually -2, but it is -1 in peroxides.

The oxidation number of a Group 1 element in a compound is +1.

The oxidation number of a Group 2 element in a compound is +2.

The oxidation number of a Group 17 element in a binary compound is -1.

The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.

The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.

EXAMPLE:

What is the oxidation number of Cr in CrCl3 ?

Solution:

We use what rules we can to determine the oxidation numbers.

Rule 7 states that the oxidation number of Cl is -1.

We write the oxidation number of the element above its symbol and the total for 3 Cl atoms below the symbol.

This gives Cr-1Cl3
mmll-3mm.

Rule 8 states the numbers along the bottom must add up to zero. So the number under Cr must be +3.

This gives Cr-1Cl3
m+3ll-3mm.

There is only one Cr atom, so its oxidation number is +3.

This gives +3Cr-1Cl3
mmmmm+3ll-3mm.

The oxidation number of Cr in CrCl3 is +3.

Here is a chart showing the oxidation numbers of the atoms in some common elements and compounds.