What is the sum of oxidation numbers for a polyatomic ion? (reiterated)

• A. The ionic charge
• B. Zero
• C. The number of atoms
• D. The total valence electrons

Answer: (A)

The sum of oxidation numbers in a polyatomic ion equals the ion’s overall charge. Oxidation numbers are assigned to electrons in bonds so that, when you add them all up across every atom in the species, the total reflects the net charge carried by the ion. For neutral molecules, that sum is zero, but for a polyatomic ion, it matches the ion’s charge.

For example, in sulfate, each oxygen is assigned −2, contributing −8 in total, so sulfur must be +6 to reach the overall −2 charge: (+6) + 4(−2) = −2. In ammonium, nitrogen is −3 and each hydrogen is +1; four hydrogens contribute +4, giving −3 + 4 = +1, which matches the ion’s +1 charge. Therefore, the sum of oxidation numbers is the ionic charge.