# What are the oxidation numbers?

Assigns the oxidation number of every element in a chemical formula or ion by the standard rules (fluorine −1, group 1 +1, group 2 +2, hydrogen ±1, oxygen −2, halogens −1) and the sum rule, with each step.

- Page: https://www.acalculator.org/chemistry/oxidation-number-calculator
- JSON spec: https://www.acalculator.org/chemistry/oxidation-number-calculator.json
- Version: 5ae15df2e297

## Default answer

Example with the default inputs (Formula KMnO4): Oxidation numbers in KMnO₄: K +1, Mn +7, O −2.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| f | Formula | A chemical formula or ion, such as KMnO4, H2O2 or Cr2O7^2-. Write a charge after ^, in superscripts, or after a space. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| result | Oxidation numbers | Each element with its oxidation number, in the order of the formula. |
| steps | Steps | The rule behind each value, and the sum rule for the element left over. |
| species | Formula | The formula as read, with subscripts and charge. |

## Method

Rules in priority order: F −1; group 1 metals +1; group 2 metals +2; H +1 with any nonmetal, −1 with metals only; O −2; Cl, Br, I, At −1 unless O, F or a halogen above them is present. The sum of all oxidation numbers equals the charge; the one element with no rule comes from it. If every element has a rule and the sum is off, the lowest-priority element comes from the sum (refused when two elements share that priority).

## Assumptions

- An element alone (O₂, S₈) is 0; an ion of one element shares its charge equally (Fe³⁺ +3, O₂²⁻ −1).
- Metals, metalloids and nonmetals as classified in OpenStax’s periodic table (metalloids: B, Si, Ge, As, Sb, Te, At).
- When an element sits in two different places in a formula (N in NH₄NO₃), the page gives only its average.
- A formula with two or more elements that no rule fixes (CuSO₄, HCN), or a wrong sum with the lowest priority shared (NaK), is refused as ambiguous.

## Worked examples

1. f = H2S gives result = H +1, S −2. Source: OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions (guidelines for assigning oxidation numbers), https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions, Example 4.5 (a): H +1, S −2.
2. f = SO3^2- gives result = S +4, O −2. Source: OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions (guidelines for assigning oxidation numbers), https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions, Example 4.5 (b): S +4.
3. f = Na2SO4 gives result = Na +1, S +6, O −2. Source: OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions (guidelines for assigning oxidation numbers), https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions, Example 4.5 (c): Na +1, S +6.
4. f = KNO3 gives result = K +1, N +5, O −2. Source: OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions (guidelines for assigning oxidation numbers), https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions, Check Your Learning after Example 4.5 (a): N +5.
5. f = AlH3 gives result = Al +3, H −1. Source: OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions (guidelines for assigning oxidation numbers), https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions, Check Your Learning after Example 4.5 (b): Al +3.
6. f = NH4^+ gives result = N −3, H +1. Source: OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions (guidelines for assigning oxidation numbers), https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions, Check Your Learning after Example 4.5 (c): N −3.
7. f = H2PO4^- gives result = H +1, P +5, O −2. Source: OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions (guidelines for assigning oxidation numbers), https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions, Check Your Learning after Example 4.5 (d): P +5.
8. f = Cr2O7^2- gives result = Cr +6, O −2. Source: LibreTexts, Jim Clark, Oxidation States (Oxidation Numbers), https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Oxidation_States_(Oxidation_Numbers): 2n + 7(−2) = −2, so chromium is +6.
9. f = H2O2 gives result = H +1, O −1. Source: LibreTexts, Jim Clark, Oxidation States (Oxidation Numbers), https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Oxidation_States_(Oxidation_Numbers): in H₂O₂ each hydrogen is +1, so each oxygen is −1.
10. f = OF2 gives result = O +2, F −1. Source: LibreTexts, Jim Clark, Oxidation States (Oxidation Numbers), https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Oxidation_States_(Oxidation_Numbers): in F₂O fluorine takes priority at −1, so oxygen is +2.
11. f = Fe3O4 gives result = Fe +8/3 (average), O −2. Source: OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions (guidelines for assigning oxidation numbers), https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions (sum rule).
12. f = KMnO4 gives result = K +1, Mn +7, O −2, species = KMnO₄. Source: OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions (guidelines for assigning oxidation numbers), https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions (sum rule).

## FAQ

### How do I find oxidation numbers?

Give each element whose value is fixed by a rule its value (fluorine −1, group 1 metals +1, group 2 metals +2, hydrogen +1, oxygen −2), then use the sum rule: the oxidation numbers of all the atoms add up to the charge. In KMnO₄, K is +1 and O is −2, so (+1) + Mn + 4 × (−2) = 0 and Mn = +7.

### What are the rules?

An element on its own is 0, and a monatomic ion has its charge. Fluorine is always −1. Group 1 metals are +1 and group 2 metals +2. Hydrogen is +1 with nonmetals and −1 with metals (hydrides such as NaH). Oxygen is −2 in most compounds. Chlorine, bromine and iodine are −1 unless they are with oxygen, fluorine or a lighter halogen. The sum equals the charge.

### Why is oxygen −1 in hydrogen peroxide?

Hydrogen’s rule comes before oxygen’s. With H at +1, two hydrogens give +2, so the two oxygens must give −2 together: −1 each. In the same way fluorine comes first in OF₂, which makes oxygen +2.

### Why does the page refuse CuSO₄?

No rule fixes copper or sulfur, so the formula alone cannot split the +6 the oxygens leave between them. Type the sulfate ion on its own, SO4^2-, to get S +6; then copper is 0 − (+6) − 4 × (−2) = +2. The page refuses rather than guesses.

### Can an oxidation number be a fraction?

Yes, as an average. In Fe₃O₄ the four oxygens give −8, so the three irons share +8: +8/3 each on average (really two Fe³⁺ and one Fe²⁺). An element in two different places, such as nitrogen in NH₄NO₃, also comes out as an average (+1 here, from −3 and +5).

### How do I type a charge?

After a caret (SO4^2-, NH4^+, Fe^3+), in superscripts (SO₄²⁻), or after a space (SO4 2-). A sign straight after a letter or bracket is a charge of 1 (OH-, Na+). A digit straight before the sign, as in Fe3+ or NO3-, could be a count or the charge, so the page asks for a caret.

## Sources

- OpenStax, Chemistry 2e, §4.2 Classifying Chemical Reactions: the guidelines for assigning oxidation numbers (elements 0, monatomic ions their charge, hydrogen +1 with nonmetals and −1 with metals, oxygen −2 with its exceptions, halogens, the sum rule), Example 4.5 (H₂S, SO₃²⁻, Na₂SO₄) and its Check Your Learning (KNO₃ N +5, AlH₃ Al +3, NH₄⁺ N −3, H₂PO₄⁻ P +5). https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions (retrieved 2026-10-19)
- LibreTexts, Jim Clark, Oxidation States (Oxidation Numbers) (CC BY-NC): group 1 metals always +1, group 2 always +2, fluorine always −1, the peroxide and F₂O exceptions with fluorine taking priority, Cr₂O₇²⁻ (Cr +6). https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Oxidation_States_(Oxidation_Numbers) (retrieved 2026-10-19)
- OpenStax, Chemistry 2e, §2.5 The Periodic Table, Figure 2.26: metals, metalloids (B, Si, Ge, As, Sb, Te, At) and nonmetals (H, He, C, N, O, F, Ne, P, S, Cl, Ar, Se, Br, Kr, I, Xe, Rn). https://openstax.org/books/chemistry-2e/pages/2-5-the-periodic-table (retrieved 2026-10-19)