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

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

Answer: (A)

Oxidation numbers are chosen so that the algebraic sum of all atoms’ oxidation numbers in a species equals the overall charge of that species. In a polyatomic ion, that total must match the ion’s charge. For example, in nitrate NO3–, oxygen is almost always –2, giving –6 from three oxygens; to reach the –1 charge of the ion, nitrogen must be +5. In sulfate SO42–, four oxygens contribute –8, so sulfur must be +6 to give the overall –2 charge. This shows that the sum of the oxidation numbers in a polyatomic ion equals the ionic charge, not zero or any other quantity like the number of atoms or the total valence electrons.