Oxidation Number Calculator
Type a chemical formula or ion to find the oxidation number of every element. The calculator applies the standard textbook rules, recognizes common polyatomic ions and coordination complexes, and shows each step so you can learn the method.
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Use correct capitals (Co = cobalt, CO = carbon monoxide). Write charges as SO4^2-, Fe^3+ or NH4+. Brackets and hydrates (· or .) are supported.
Only needed if the charge is not written in the formula
How to use the Oxidation Number Calculator
- Type the formula with correct capital letters, for example K2Cr2O7, Fe2(SO4)3 or [Co(NH3)6]Cl3.
- Add the charge for an ion, either in the formula (SO4^2-, NH4+) or in the Charge box.
- Press Find oxidation numbers, or pick one of the example chips.
- Read the result: each element's oxidation number, with positive values in red and negative values in blue.
- Follow the steps to see which rule set each value and how the charge balance was checked.
Rules for assigning oxidation numbers
The calculator applies these rules in order of priority. When a rule conflicts with a higher one, the higher rule wins and the lower element is solved from the charge balance.
- Free elements (Na, O₂, S₈, Fe) have an oxidation number of 0. A monatomic ion has an oxidation number equal to its charge (Fe³⁺ is +3).
- The sum of all oxidation numbers equals 0 for a neutral compound and the ion's charge for an ion.
- Fluorine is always −1 in compounds.
- Group 1 metals (Li, Na, K, Rb, Cs) are +1 and group 2 metals (Be, Mg, Ca, Sr, Ba) are +2. Aluminum is +3 and zinc is +2.
- Hydrogen is +1 with nonmetals and −1 in hydrides with metals or boron and silicon.
- Oxygen is −2, except in peroxides (−1), superoxides (−1/2) and fluorides (positive).
- Halogens (Cl, Br, I) are −1 unless bonded to oxygen or a more electronegative halogen.
- Everything else is solved from the charge balance. If several elements remain, the most electronegative one takes its usual negative value (N −3, S −2, C −4) first.
Oxidation numbers in common ions
| Ion | Name | Central atom | Oxidation number |
|---|---|---|---|
| SO₄²⁻ | sulfate | S | +6 |
| SO₃²⁻ | sulfite | S | +4 |
| S₂O₃²⁻ | thiosulfate | S | +2 (average) |
| NO₃⁻ | nitrate | N | +5 |
| NO₂⁻ | nitrite | N | +3 |
| NH₄⁺ | ammonium | N | −3 |
| PO₄³⁻ | phosphate | P | +5 |
| CO₃²⁻ | carbonate | C | +4 |
| C₂O₄²⁻ | oxalate | C | +3 |
| MnO₄⁻ | permanganate | Mn | +7 |
| Cr₂O₇²⁻ | dichromate | Cr | +6 |
| ClO₄⁻ / ClO₃⁻ / ClO⁻ | perchlorate / chlorate / hypochlorite | Cl | +7 / +5 / +1 |
| CN⁻ | cyanide | C (N is −3) | +2 |
Worked example: Fe in K₄[Fe(CN)₆]
Potassium is a group 1 metal, so the four K atoms contribute +4. The complex ion [Fe(CN)₆] must therefore carry −4. Each cyanide ligand is CN⁻ (−1), so six of them give −6, and iron must be −4 − (−6) = +2. Inside cyanide, nitrogen is the more electronegative atom and takes −3, leaving carbon at +2.
Limits of the rules
- Averages hide structure. In NH₄NO₃ the calculator recognizes both ions, but in a formula like CH₃CN the two carbons (−3 and +3) can only be shown as an average of 0.
- Bonds between like atoms (peroxides inside larger molecules, disulfides such as FeS₂) need the structure to assign correctly.
- Some metals can take several states (Cu⁺/Cu²⁺, Fe²⁺/Fe³⁺). When the formula alone cannot decide, the calculator says which value it assumed.
Frequently asked questions
How do you find oxidation numbers?
Start from the rules that never change: free elements are 0, group 1 metals are +1, group 2 metals are +2, fluorine is −1, hydrogen is usually +1 and oxygen usually −2. Then use the fact that all oxidation numbers add up to the overall charge to solve for the remaining element.
What is the oxidation number of Cr in K2Cr2O7?
Potassium is +1 and oxygen is −2. The total must be zero: 2(+1) + 2x + 7(−2) = 0, so 2x = 12 and chromium is +6.
What is the oxidation number of sulfur in SO4^2-?
Oxygen is −2, and the four oxygens add up to −8. The ion has a charge of −2, so sulfur must be +6.
Can an oxidation number be a fraction?
An individual atom always has a whole-number oxidation state, but when atoms of the same element are in different environments, the formula-based method gives their average. In Fe3O4 the average is +8/3 because the crystal contains one Fe²⁺ and two Fe³⁺ ions; in S4O6²⁻ sulfur averages +2.5.
When is oxygen not −2?
Oxygen is −1 in peroxides such as H2O2 and Na2O2, −1/2 in superoxides such as KO2, and positive when bonded to fluorine, for example +2 in OF2. In elemental O2 and O3 it is 0.
When is hydrogen −1?
Hydrogen is −1 in metal hydrides, where it is bonded to elements less electronegative than itself, such as NaH, CaH2, LiAlH4 and NaBH4. With nonmetals like carbon, nitrogen and oxygen it is +1.
What is the difference between oxidation number and charge?
Charge belongs to a whole ion. Oxidation numbers are bookkeeping charges assigned to each atom as if all bonds were ionic, and they always add up to the overall charge. They are used to track electron transfer in redox reactions.
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