Oxidation Number Calculator
Type a formula or ion and get the oxidation number of every element, the rule that decided it, and a check that the numbers add up to the charge. Peroxides, superoxides, hydrides and the fraction-valued averages of mixed-valence and organic compounds are flagged instead of being quietly guessed. Runs entirely in your browser. Free, no sign-up.
Case matters: Co is cobalt, CO is carbon monoxide. Write a charge as SO4^2-, Fe^3+ or MnO4-. Brackets and hydrates work: K4[Fe(CN)6], CuSO4·5H2O.
Results
K₂Cr₂O₇
| Element | Atoms | Oxidation number | Stock notation | Rule used |
|---|---|---|---|---|
| Potassium (K) | 2 | +1 | K(I) | Group 1 metals are +1. |
| Chromium (Cr) | 2 | +6 | Cr(VI) | Found from the sum: the numbers add up to the charge of the species. |
| Oxygen (O) | 7 | −2 | O(−II) | Oxygen is −2 in most compounds. |
2 × (+1) + 2 × (+6) + 7 × (−2) = 0
Oxidation numbers are bookkeeping, not real charges: they show where the bonding electrons are assigned, which is what you need to follow a redox reaction.
How to Use the Oxidation Number Calculator
- 1. Type the formula. Write it the usual way: H2SO4, Ca(OH)2, Fe2(SO4)3. Case matters, because Co is cobalt and CO is carbon monoxide.
- 2. Add the charge for an ion. Use a caret or a trailing sign: SO4^2-, MnO4-, Fe^3+. Brackets and hydrates work too, such as K4[Fe(CN)6] and CuSO4·5H2O.
- 3. Read the table. Each element gets its oxidation number, its Stock notation (such as Cr(VI)) and the rule that decided it. Compounds with polyatomic ions are shown ion by ion.
- 4. Check the flags. Any exception, such as a peroxide, a hydride or an average, is explained under the table, so you know when a rule of thumb has run out.
The Rules, in Priority Order
An oxidation number is the charge an atom would have if its bonds were treated as fully ionic: each bonding pair of electrons goes to the more electronegative atom, and is split equally between identical atoms. The textbook rules below reproduce that bookkeeping. When two rules conflict, the one higher in the list wins:
- An atom in an element has oxidation number 0, and a monatomic ion has a number equal to its charge.
- The numbers of all atoms in a species add up to its charge: 0 for a neutral compound.
- Fluorine is −1 in every compound.
- Group 1 metals are +1 and group 2 metals are +2.
- Hydrogen is +1, except −1 when bonded only to metals or boron (a hydride).
- Oxygen is −2, except −1 in peroxides, −½ in superoxides and positive when bonded to fluorine.
- Chlorine, bromine and iodine are −1, except when combined with oxygen, fluorine or a more electronegative halogen.
- The one remaining element takes whatever value makes the sum equal the charge.
The order of the first, second, hydrogen, oxygen and halogen rules follows OpenStax Chemistry 2e; the group 1 and 2 rules are added because they settle cases such as NaH and KO₂. The calculator follows the same order. Oxidation numbers are the basis for spotting what is oxidised and what is reduced in a reaction: the element whose number rises is oxidised. The Chemical Equation Balancer balances the equation once you know.
Worked Example: Potassium Dichromate
The calculator opens on K₂Cr₂O₇. Potassium is in group 1, so it is +1, and oxygen is −2. Chromium is the only element left, so the sum rule gives 2(+1) + 2x + 7(−2) = 0, which gives x = +6. In Stock notation, chromium is Cr(VI), and that is why dichromate is a strong oxidising agent: it is ready to drop to Cr(III).
Exceptions the Calculator Flags
| Formula | Result | Why |
|---|---|---|
| H₂O₂ | H +1, O −1 | Peroxide: only an O–O bond makes the sum work |
| KO₂ | K +1, O −1/2 | Superoxide: a fractional average over O₂⁻ |
| NaH | Na +1, H −1 | Hydride: hydrogen bonded only to a metal |
| OF₂ | O +2, F −1 | Fluorine outranks oxygen |
| Fe₃O₄ | Fe +8/3 | Average of two Fe(III) and one Fe(II) |
| NH₄NO₃ | N −3 and N +5 | Two different nitrogens, shown ion by ion |
| C₆H₁₂O₆ | C 0 (average) | Each carbon has its own number |
Two or more elements without a fixed rule need extra information. The calculator first recognises common polyatomic ions such as sulfate, nitrate, carbonate and permanganate, so FeSO₄ becomes Fe²⁺ and SO₄²⁻ with iron at +2. If that fails it lets the more electronegative of two elements take its usual negative number, which settles CS₂ and SiC. When even that cannot decide, it says so rather than guess.
Averages are not individual numbers
For an organic compound the calculator gives the average over all carbon atoms, because the textbook rules cannot see the structure. In methanol, CH₃OH, there is one carbon and the answer, −2, is exact; in ethanol each carbon has its own number (−3 and −1), and the average, −2, hides that. Draw the Lewis structure and assign each atom's bonds to see the real values.
Frequently Asked Questions
- How do you find the oxidation number of an element in a compound?
- Assign the elements that have fixed numbers first (fluorine −1, group 1 +1, group 2 +2, hydrogen usually +1, oxygen usually −2), then give the remaining element the number that makes the total equal the charge. In H₂SO₄, 2(+1) + x + 4(−2) = 0 gives sulfur +6.
- What is the oxidation number of oxygen in a peroxide?
- It is −1, not −2. In a peroxide each oxygen is bonded to another oxygen, an identical atom, so that bond is split equally and counts for nothing; each oxygen takes electrons only from its other bond, which gives −1. Hydrogen peroxide, H₂O₂, and sodium peroxide, Na₂O₂, are the standard examples.
- Can an oxidation number be a fraction?
- Yes, as an average. In Fe₃O₄ the three iron atoms are Fe(II) and Fe(III) together, which averages to +8/3, and in superoxides oxygen is −½. Each individual atom still has a whole-number state in a real structure; the fraction is the mean, which the calculator flags.
- What is the difference between oxidation number and charge?
- Charge is physical, for example +2 on a Fe²⁺ ion. An oxidation number is a bookkeeping device that assigns bonding electrons to the more electronegative atom, so in covalent molecules like CH₄ the −4 on carbon is not a real charge. For a simple monatomic ion the two coincide.
- How do I tell which species is oxidised and which is reduced?
- Find the oxidation number of each element before and after the reaction. The element whose number goes up is oxidised, and the one whose number goes down is reduced. Run the reactants and products through the calculator and compare, for example Mn +7 in MnO₄⁻ falling to +2 in Mn²⁺.
- How do you find the oxidation number of carbon in an organic molecule?
- Count each bond to a more electronegative atom (O, N, halogen) as +1, each bond to hydrogen as −1, and ignore bonds to other carbons. The calculator gives the average over all carbons from the formula alone, which is exact only when the molecule has a single carbon or all carbons are alike.