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Oxidation Number Calculator

Determine the oxidation state of each atom in a chemical compound, ion, or molecule.

Oxidation Number Calculator

Enter a formula. For ions, append charge: SO4^2-, Fe^3+, MnO4^-
Net charge (0 for neutral molecules)

How It Works

What is an Oxidation Number?

An oxidation number (or oxidation state) is the hypothetical charge an atom would have if all bonds were completely ionic. It helps track electron transfer in redox reactions.

Rules for Assigning Oxidation Numbers
Rule Description Example
1 Free elements have oxidation number 0 O₂, Fe, N₂ = 0
2 Monoatomic ions equal their charge Na⁺ = +1, Cl⁻ = -1
3 Hydrogen is usually +1 H₂O: H = +1
4 Oxygen is usually -2 H₂O: O = -2
5 Hydrogen is -1 with metals (metal hydrides) NaH: H = -1
6 Oxygen is -1 in peroxides H₂O₂: O = -1
7 Fluorine is always -1 HF: F = -1
8 Group IA metals are +1, Group IIA are +2 NaCl: Na = +1
9 Sum of oxidation numbers equals overall charge SO₄²⁻: sum = -2
Importance in Chemistry
  • Redox Reactions: Identify which atoms are oxidized (increase) and reduced (decrease)
  • Balancing Equations: The half-reaction method uses oxidation numbers to balance redox equations
  • Nomenclature: Roman numerals in compound names indicate oxidation states (e.g., Iron(III) chloride)
  • Electrochemistry: Oxidation numbers help predict cell potentials and electrode reactions
Note: This calculator applies standard rules in a fixed priority order. For unusual compounds (e.g., superoxides, OF₂), the assigned states follow the most common conventions.

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