Example Five

What is the oxidation number of Carbon in CaCO₃?

SOLUTION: Since CaCO₃ is a compound, we are at the third box in the chart. We'll deal first with the calcium and oxygen, since we can be reasonably sure of their oxidation numbers.

1. Calcium is in Group II, so calcium's oxidation state is +2.
   
   
2. There are three oxygen atoms, so the total for oxygen is -6 (3 X -2 = -6)
   
   
3. If we add the calcium and the oxygen oxidation numbers together, we get -4 (+2 + -6 = -4). Since all we have left is one carbon atom, it must have a +4 oxidation number to get a total of zero.

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