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

Analyze the question The question asks for the total of all the oxidation numbers of the atoms in a compound that has no overall electric charge.

Concept Oxidation number is the charge an atom would have if all its bonds were treated as fully ionic. A key rule is:

• In a neutral compound, the sum of the oxidation numbers of all atoms = 0.
• In a polyatomic ion, the sum equals the charge on the ion.

Worked examples

1. H₂O: H = +1 (two atoms) and O = -2 Sum = 2(+1) + (-2) = 0

2. NaCl: Na = +1 and Cl = -1 Sum = +1 + (-1) = 0

3. H₂SO₄: H = +1 (two atoms), O = -2 (four atoms), S = +6 Sum = 2(+1) + (+6) + 4(-2) = 2 + 6 - 8 = 0

In every case the positive and negative oxidation numbers cancel exactly, because the compound has no net charge.

Answer The sum is 0, which is option C.

Why the others are wrong

• A (+2), B (-1) and D (+1) would each mean the compound carries a net charge, so it would not be neutral. A sum of -1 or +1 would apply to an ion such as Cl⁻ or Na⁺, not a neutral compound.

Exam tip Remember: neutral compound → sum is zero; ion → sum equals its charge. You can use this to find an unknown oxidation number, for example S in H₂SO₄: 2(+1) + S + 4(-2) = 0, so S = +6.