Assigning Oxidation Numbers

COMPOUNDS

5.10 - Oxidation Numbers

What is the oxidation number of sulfur in H₂SO₄?

The oxidation number of hydrogen is +1 (Rule #3).
There are two hydrogen atoms.
The oxidation number of oxygen is -2 (Rule #6).
There are four oxygen atoms.
The overall charge of the compound is zero, so the sum of the oxidation numbers of all the atoms in the formula is zero (Rule #8).

Let the oxidation number of sulfur be represented as S. Set up the equation and solve for S.

2(+1) + 4(-2) + S = 0

-6 + S = 0

S = +6

Or, you can just consider the SO₄^2- ion:

Then, you must apply Rule #9

Rule #9:

The sum of oxidation numbers of an ion or complex ion is the same as the charge on that ion.

Let the oxidation number of sulfur be represented as S. Set up the equation and solve for S.

4(-2) + S = -2

-8 + S = -2

S = +6

Either way, you should still come out with an oxidation number of +6 for sulfur.

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