In an ion, the sum of the oxidation numbers equals what value?

• A. 0
• B. The charge on the ion
• C. +1
• D. The sum of the oxidation numbers in the ion equals the ion's charge

Answer: (D)

When you assign oxidation numbers, you’re keeping track of electrons as if they are transferred, and the numbers you assign to each atom sum to the overall charge of the species. For a neutral molecule, that sum is zero. For an ion, the total must balance the ion’s net charge, so the algebraic sum of all oxidation numbers equals the ion’s charge.

This is clear with examples: in ammonium NH4+, hydrogen is +1 for each of the four hydrogens, contributing +4; to give the overall +1 charge, nitrogen must be -3, so the total is -3 + 4 = +1. In sulfate SO4^2-, each oxygen is -2 (total -8); sulfur must be +6 to yield -2 overall, so the sum is +6 + (-8) = -2. The general rule is that the sum of oxidation numbers equals the ion’s charge, not a fixed value like zero or +1 for all ions.