What is the sum of the oxidation numbers in a neutral compound?

• A. 0
• B. The charge on the ion
• C. +1
• D. -1

Answer: (A)

Oxidation numbers are assigned so that their algebraic sum equals the overall charge of the species. For a neutral compound, that charge is zero, so the oxidation numbers must add to zero. For example, in water each hydrogen is +1 and the oxygen is -2, giving (+1) + (+1) + (-2) = 0. In sodium chloride, sodium is +1 and chlorine is -1, summing to zero. In carbon dioxide, carbon is +4 and each oxygen is -2, giving +4 + (-2) + (-2) = 0. This is why the sum in a neutral compound is zero. The other possibilities describe the charge of ions, not neutral substances, so they don’t apply here.