🔢 Oxidation Number Calculator
Enter a formula and get the oxidation number of every element — the tool applies the standard rules and solves the unknown by balancing to the overall charge.
K × 1
+1
O × 4
-2
Mn × 1
+7
solved
Oxidation numbers sum to the overall charge (0). Standard rules fixed the others; Mn was solved by balance.
Uses standard rules (group 1 = +1, group 2 = +2, F = −1, O = −2, H = +1, …) and solves the remaining element by charge balance. Peroxides, metal hydrides and O–F compounds are exceptions. 🔒 In your browser.
How the oxidation number calculator works
The tool parses the formula and assigns oxidation states by the standard rules (group 1 = +1, group 2 = +2, fluorine = −1, oxygen = −2, hydrogen = +1, and so on). The remaining element is found by requiring the oxidation numbers to sum to the overall charge (0 for a neutral compound). For KMnO₄, K = +1 and O = −2 fix everything but manganese, which solves to +7.
This handles the common textbook cases exactly. Watch the well-known exceptions — peroxides (O = −1), metal hydrides (H = −1) and oxygen–fluorine compounds — where the default rules don’t apply.
Frequently asked questions
How do I find the oxidation number of an element?
Assign the elements with fixed rules (group 1 = +1, O = −2, H = +1, etc.), then solve the remaining one so all the oxidation numbers sum to the overall charge. In KMnO₄, +1 + Mn + 4(−2) = 0 gives Mn = +7.
What are the oxidation number rules?
Free elements are 0; group 1 metals +1; group 2 +2; fluorine −1; oxygen usually −2; hydrogen usually +1; and the sum of oxidation numbers equals the ion’s charge (0 for a neutral compound).
Can it handle polyatomic ions?
Yes — enter the formula and set the overall charge (e.g. −1 for a nitrate-like ion). The balance then solves for the central atom.
What are the main exceptions?
Peroxides give oxygen −1 (as in H₂O₂), metal hydrides give hydrogen −1 (as in NaH), and in OF₂ oxygen is +2. The tool uses the default rules, so treat these cases specially.
Why can’t some formulas be solved?
If two or more elements lack a fixed rule, the oxidation states aren’t uniquely determined by rules alone, and you need additional chemistry (like known ion charges) to assign them.