In a polyatomic ion, the sum of the oxidation numbers of all atoms equals what?

• A. Sum equals zero
• B. Sum equals +1
• C. Sum equals -1
• D. Sum equals the ion's charge

Answer: (D)

In a polyatomic ion, the oxidation numbers of all atoms add up to the ion’s overall charge. This bookkeeping rule comes from the way oxidation states are assigned to atoms (for example, oxygen is usually -2 per atom, hydrogen +1, etc.). When you sum the oxidation numbers across every atom in the ion, the total must match the net charge the ion carries. For instance, in sulfate, four oxygens contribute -8, so sulfur must be +6 to give an overall charge of -2. This illustrates that the total is not zero unless the species is neutral, but a polyatomic ion is by definition charged, so its oxidation numbers sum to that charge. Therefore, the sum of the oxidation numbers equals the ion’s charge.