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Periodixy

Oxidation Number Calculator

Enter a compound or ion (write charges like SO4^2- or NH4+) and get each element's oxidation number, the rule that fixed it, and a sum check against the overall charge. Cases the rules can't settle — like mixed-valence Fe₃O₄ — are flagged honestly instead of guessed.

Try:

The rules, in the order they apply

  1. Free elements are 0 (O₂, Fe, S₈).
  2. Monatomic ions equal their charge (Na⁺ = +1).
  3. Fluorine is always −1 in compounds.
  4. Group 1 metals +1; group 2 metals +2; Al +3.
  5. Hydrogen +1 with nonmetals; −1 in metal hydrides (NaH).
  6. Oxygen −2, except peroxides (−1) and OF₂ (+2).
  7. Everything must sum to the overall charge — which usually pins down the one remaining unknown.

Worked example: KMnO₄

Find manganese's oxidation number in potassium permanganate.

  1. K = +1 (group 1). Four O = 4 × (−2) = −8.
  2. Sum must be 0: (+1) + Mn + (−8) = 0.

Answer: Mn = +7 — manganese's maximum, making KMnO₄ a powerful oxidiser.

What oxidation numbers are for

They track electrons through redox reactions: an atom whose oxidation number rises was oxidised (lost electrons), one whose number falls was reduced (gained them) — OIL RIG. Balancing redox equations, understanding batteries, and explaining corrosion all start from this bookkeeping. The full rule set with more examples is in Oxidation Numbers Explained.

⚠ Common mistake: Oxidation numbers are a convention, not real charges — except in genuinely ionic compounds. Don't read Mn(+7) as a physical 7+ charge on one atom.

Frequently asked questions

How do I enter ions?

Add the charge at the end: SO4^2-, NH4+, PO4^3-, Fe3+ (a lone element with a charge is treated as a monatomic ion).

Why does Fe3O4 give an error?

Magnetite contains iron in two different states (one Fe²⁺ + two Fe³⁺), so no single whole number describes “the” iron. The calculator flags such mixed-valence cases for manual analysis rather than reporting a misleading average.

What is the oxidation number of oxygen in H2O2?

−1. Hydrogen peroxide is a peroxide — one of the standard exceptions to oxygen's usual −2.

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Learn the concepts

Oxidation Numbers ExplainedLearn the oxidation number rules — free elements are 0, oxygen −2, hydrogen +1 — and use them to solve KMnO₄, SO₄²⁻ and more, step by step.What Are Ions?Ions are charged atoms formed by losing or gaining electrons. Learn cations vs anions, how to predict charges from the periodic table, and polyatomic ions.