Oxidation Numbers Explained
By the Periodixy Editorial Team · Last reviewed July 10, 2026
Oxidation numbers are a bookkeeping system for electrons. They assign each atom in a compound a hypothetical charge, as if every bond were fully ionic. With them you can track which atoms lose electrons (oxidation) and which gain them (reduction) in any redox reaction.
The rules, in priority order
- Free elements are 0 (O₂, Fe, P₄ — all 0).
- Monatomic ions equal their charge (Na⁺ = +1, S²⁻ = −2).
- Fluorine is always −1 in compounds.
- Group 1 metals are +1; group 2 metals are +2; aluminium is +3.
- Hydrogen is +1 with nonmetals, −1 in metal hydrides (NaH).
- Oxygen is −2 — except peroxides (H₂O₂: −1) and with fluorine (OF₂: +2).
- The sum of all oxidation numbers equals the overall charge (0 for neutral compounds).
Worked examples
Manganese in KMnO₄
Find the oxidation number of Mn in potassium permanganate.
- K is group 1 → +1. Each O → −2 (four of them: −8).
- Sum must be 0: (+1) + Mn + (−8) = 0.
- Mn = +7.
Answer: Mn is +7 — its highest possible state, which is why KMnO₄ is such a strong oxidiser.
Sulfur in the sulfate ion
Find S in SO₄²⁻.
- Four O at −2 → −8.
- Sum must equal the ion's charge, −2: S + (−8) = −2.
- S = +6.
Answer: S is +6 in sulfate.
The Oxidation Number Calculator applies these exact rules and shows which one fixed each atom.
Using oxidation numbers in redox
In a redox reaction, oxidation numbers change. The atom whose number increases was oxidised (lost electrons); the one whose number decreases was reduced (gained electrons). Mnemonic: OIL RIG — Oxidation Is Loss, Reduction Is Gain.
Spotting the redox pair
In Zn + Cu²⁺ → Zn²⁺ + Cu, what is oxidised and what is reduced?
- Zn goes 0 → +2: increased → oxidised.
- Cu goes +2 → 0: decreased → reduced.
Answer: Zinc is oxidised; copper(II) is reduced. This is the classic zinc-copper battery reaction.
Where the simple rules break
Mixed-valence compounds like Fe₃O₄ (one Fe²⁺ + two Fe³⁺) give fractional averages; organic molecules need carbon-by-carbon analysis; and unusual peroxides or superoxides bend the oxygen rule. For schoolwork, the seven rules above cover nearly every case you will meet.
Summary
- Oxidation numbers are assigned charges for tracking electrons.
- Key rules: free element 0, F −1, O −2 (peroxide −1), H +1 (hydride −1), sum = overall charge.
- Solve for unknowns with the sum rule.
- Increase = oxidised; decrease = reduced (OIL RIG).