Oxidation Number Calculator
Type a formula or ion. Write charges with a caret: SO4^2-, NH4^+.
| Element | Atoms | Oxidation no. | Contribution | Why |
|---|---|---|---|---|
| K | 1 | +1 | +1 | Group 1 metals are +1. |
| Mn | 1 | +7 | +7 | Solved so the sum equals the overall charge (0). |
| O | 4 | −2 | −8 | Oxygen is −2 (except in peroxides and with fluorine). |
| Sum | 0 | must equal the overall charge (0) | ||
For organic molecules the result is the average over all carbons. Individual carbons can differ.
Ask about oxidation states
AI answers can be wrong. The calculators on this page are deterministic; trust them for numbers.
Working it by hand
Take potassium permanganate, KMnO₄. Potassium is a group 1 metal: +1. Oxygen is −2, four of them make −8. The compound is neutral, so Mn + 1 − 8 = 0, and manganese is +7. For the dichromate ion Cr₂O₇²⁻: 7 × (−2) = −14, the total must be −2, so the two chromiums share +12, which is +6 each.
Common charges for simple ions are on the periodic table with charges.
Questions students ask
What are the rules for assigning oxidation numbers?
1) A free element is 0. 2) A monatomic ion equals its charge. 3) Fluorine is always −1. 4) Group 1 metals are +1, group 2 are +2, aluminium +3. 5) Hydrogen is +1 with nonmetals and −1 in metal hydrides. 6) Oxygen is −2, except −1 in peroxides and positive with fluorine. 7) The sum over all atoms equals the overall charge (0 for a neutral compound).
Why do I get a fractional oxidation number?
The value is an average. In Fe₃O₄ one iron is +2 and two are +3, averaging +8/3. In the tetrathionate ion S₄O₆²⁻ sulfur averages +2.5.
How are oxidation numbers used in redox?
An element whose oxidation number goes up is oxidised (loses electrons); one that goes down is reduced (gains electrons). Comparing numbers before and after tells you which species is the oxidising agent and which the reducing agent.
Is an oxidation number the same as a formal charge?
No. Oxidation numbers assume every bond is fully ionic, giving shared electrons to the more electronegative atom. Formal charge assumes bonds are shared equally. They answer different questions.