Oxidation Number Calculator
Enter a formula or ion and get the oxidation number of every atom. The standard rules are applied to the elements that have one, then the remaining element is solved so the oxidation numbers sum to the overall charge — with the reasoning shown for each.
How to use this tool
Type a neutral compound (KMnO4) or a polyatomic ion (MnO4-, Cr2O7^2-). The tool fixes the oxidation number of every element that follows a rule (alkali +1, O −2, H +1…), then back-solves the one element left over.
What to enter
- Formula: element symbols with counts — case matters (Co cobalt vs CO carbon monoxide).
- Charge: append it for ions — MnO4-, SO4^2-, Cr2O7^2-, Fe3+. One level of parentheses is supported.
Reading the result
The table gives each element's oxidation number and why. The sum check confirms the numbers add up to the species charge. A non-integer value (e.g. Fe in Fe3O4) is the average across mixed-valence sites. Peroxides, superoxides and metal hydrides break the simple rules and are flagged rather than guessed.
Worked example
In KMnO4: K is +1 and O is −2 by rule, so Mn = 0 − (1 + 4·(−2)) = +7.
Formula or ion
Oxidation numbers
Each element's oxidation number plus the rule (or the solve) behind it. The sum of oxidation number × count equals the species charge.
Methodology
Oxidation numbers are assigned by the conventional priority rules, then the single remaining element is found from the constraint that the sum of (oxidation number × atom count) equals the overall charge of the species.
Rules applied (in order)
- A free element is 0; a monatomic ion equals its charge.
- Fluorine is always −1.
- Group 1 metals +1; Group 2 metals +2.
- Hydrogen +1 (−1 only in metal hydrides such as NaH).
- Oxygen −2 (−1 in peroxides, −½ in superoxides, +2 in OF₂).
- Cl, Br, I are −1 when no oxygen or fluorine is present; otherwise they are solved.
- The remaining element is solved from the charge balance.
Sources
- IUPAC Recommendations on the oxidation state / oxidation number.
- Standard general-chemistry oxidation-number rules (Brown, LeMay; Petrucci).
Known limits
- Exceptions that depend on structure rather than formula — peroxides, superoxides, metal hydrides — are detected as an inconsistent sum and flagged, not silently mis-assigned.
- A formula with two or more elements that lack a fixed rule is under-determined and reported as such.
- Mixed-valence solids (e.g. Fe₃O₄) return the average oxidation number, not the individual site values.