Example Four

What is the oxidation number of Sulfur in SO₄^2-?

SOLUTION: Since SO₄^2- contains two different atoms, we are at the third box in the chart. Note that this is a polyatomic ion (an ion with several atoms). We will be trying to get the oxidation numbers to add up to -2 this time, not zero.

1. There are four oxygen atoms, each with a -2 oxidation number. The total for the oxygen atoms is -8 (4 X -2 = -8).
   
   
2. Since we want the total to be -2, the sulfur must have a +6 oxidation number (-8 + +6 = -2).

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