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Oxidation Number Calculator

Enter a formula or ion to get the oxidation state of each element, with the rule used at every step. It handles peroxides, metal hydrides, OF2, polyatomic ions and fractional (average) oxidation states.

Quick answer: in K2Cr2O7, K = +1 and O = −2, so 2(+1) + 2x + 7(−2) = 0 and Cr = +6.

Write ion charges at the end: SO4^2-, NH4+, Fe3+.

Results appear here.

How to use the oxidation number calculator

  1. Type a neutral formula (KMnO4) or an ion with its charge (Cr2O7^2-, NH4+).
  2. The calculator applies the oxidation-number rules in priority order: F, then Group 1, Group 2, Al, H, O and the halogens.
  3. The element with no fixed rule is found by making the sum of oxidation numbers equal the overall charge.
  4. Read the table and the numbered steps. Fractional answers are averages over atoms in different environments.

Worked examples

Potassium permanganate, KMnO4

K = +1, O = −2
(+1) + x + 4(−2) = 0

Mn = +7

Hydrogen peroxide, H2O2

H = +1, so 2(+1) + 2x = 0

O = −1 (peroxide)

Dichromate ion, Cr2O72−

O = −2, total charge −2
2x + 7(−2) = −2

Cr = +6

Magnetite, Fe3O4

3x + 4(−2) = 0

Fe = +8/3 (average of one Fe2+ and two Fe3+)

Rules for assigning oxidation numbers

  1. An atom in an element (Na, O2, S8) is 0.
  2. A monatomic ion equals its charge (Fe3+ = +3, Cl− = −1).
  3. Fluorine is always −1 in compounds.
  4. Group 1 metals are +1, Group 2 metals are +2 and aluminium is +3.
  5. Hydrogen is +1 with non-metals and −1 in metal hydrides (NaH, CaH2, LiAlH4).
  6. Oxygen is −2, except in peroxides (−1), superoxides (−½) and OF2 (+2).
  7. Cl, Br and I are −1 unless bonded to oxygen or a more electronegative halogen.
  8. The sum of all oxidation numbers equals the overall charge: 0 for a compound, the ion charge for an ion.

The calculator follows these rules in that order, which is how exceptions such as peroxides fall out naturally: in H2O2 hydrogen is fixed first, so oxygen has to be −1 to make the sum zero.

Oxidation, reduction and identifying Fe2+ vs Fe3+

An increase in oxidation number is oxidation; a decrease is reduction (OIL RIG). To tell whether iron is +2 or +3 in a compound, balance the charges of the other ions: in FeCl3, three Cl− means Fe3+; in FeSO4, SO42− means Fe2+. Going from Fe2+ to Fe3+ is oxidation.

Limitations. Rules give one value per element. When the same element sits in different environments (NH4NO3, organic molecules, S4O62−), the result is an average. Salts are also split into their ions where possible. For organic carbon atoms, draw the structure and assign each carbon from its bonds.

Working on redox mechanisms? Draw structures with formal charges and curved arrows in the free MolDraw editor.

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Frequently asked questions

How do you know an oxidation number?

Apply the fixed rules (F −1, Group 1 +1, Group 2 +2, H +1, O −2, halogens −1) and solve for the remaining element so that the sum equals the overall charge.

How do you find the oxidation state of Cr in K2Cr2O7?

K is +1 and O is −2: 2(+1) + 2x + 7(−2) = 0, so 2x = 12 and Cr = +6. In the ion Cr2O7 2− the answer is the same: 2x − 14 = −2.

How do I know if Fe is 2+ or 3+?

Use the charges of the other ions. FeCl3 needs Fe3+ to balance three Cl−; FeO and FeSO4 contain Fe2+. Fe3O4 contains both, so the average is +8/3.

Is Fe2+ to Fe3+ oxidation?

Yes. The oxidation number increases from +2 to +3 because iron loses one electron, and that is oxidation.

Why is the oxidation number of oxygen −1 in H2O2?

Hydrogen is +1, so two H contribute +2. The two oxygens must total −2, giving −1 each. The O–O bond does not change oxidation numbers.

Can oxidation numbers be fractions?

Yes, as averages. Fe3O4 gives +8/3 and S4O6 2− gives +2.5, because atoms of the same element are in different oxidation states.

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