About the Oxidation Number Calculator
This oxidation number calculator assigns an oxidation state to every element in a chemical formula or polyatomic ion using the standard textbook rules: free elements are 0, fluorine is −1, group 1 and 2 metals are +1 and +2, hydrogen is +1 (−1 in metal hydrides), oxygen is −2 (−1 in peroxides), and the sum of all oxidation numbers equals the overall charge. The remaining element is solved from that charge balance.
It is useful for balancing redox equations, naming transition-metal compounds (iron(III) oxide, manganese(VII) in permanganate) and checking homework. Type ions with a caret or trailing sign — SO4^2-, MnO4-, NH4+, Fe3+ — or enter the charge separately. For a single element followed by a number and a sign (Fe3+, O2-) the number is read as the charge. Brackets and hydrates such as K4[Fe(CN)6] and CuSO4·5H2O are understood.
When an element appears in more than one environment (for example the two nitrogens in NH4NO3 or the sulfurs in S4O6^2-) the result is the average oxidation state, which can be a fraction. Rules are applied in priority order and relaxed only when they cannot all hold, which is how peroxides (H2O2) and superoxides (KO2) are detected.
How to use the oxidation number calculator
- 1Type the formula with correct capitalisation (KMnO4, not kmno4).
- 2For an ion, add the charge: SO4^2-, NH4+, or use the charge box.
- 3Read the oxidation state of every element.
- 4Check the table to see which rule set each value.
- 5Confirm the charge check matches the overall charge.
Formula and method
Oxidation numbers are bookkeeping charges assigned as if every bond were ionic, with shared electrons given to the more electronegative atom. The calculator applies the rules in priority order: free elements 0; fluorine −1; group 1 metals +1; group 2 metals +2; aluminum +3 and zinc +2; hydrogen +1 with nonmetals but −1 in hydrides of metals, boron and silicon (NaH, NaBH4, SiH4); oxygen −2 unless bonded to fluorine; and Cl, Br or I −1 when they are the most electronegative element present.
The element left over is solved so that the oxidation numbers multiplied by atom counts add up to the charge. If two or more unknowns remain, the most electronegative nonmetal is given its usual negative state (N −3, S −2, C −4). If the fixed rules contradict the charge, the lowest-priority rule is relaxed first, so oxygen becomes −1 in peroxides and −½ in superoxides.
- Σ
- Sum over every element in the formula
- overall charge
- 0 for a neutral compound, or the ion charge (e.g. −2 for SO4^2-)
Worked examples
Potassium permanganate, KMnO4
K is +1 (group 1) and each O is −2, contributing −8. For a neutral compound +1 + Mn − 8 = 0, so manganese is +7 — hence the name manganese(VII).
Dichromate ion, Cr2O7^2-
Seven oxygens give 7 × (−2) = −14. The two chromium atoms must total −2 − (−14) = +12, so each Cr is +6.
Hydrogen peroxide, H2O2
With H = +1 and O = −2 the total would be −2, not 0, so the oxygen rule is relaxed. Two H atoms give +2, so each O must be −1, the signature of a peroxide.
Ammonium ion, NH4+
Four hydrogens at +1 give +4. To reach the +1 charge of the ion, nitrogen must be −3.
Monatomic ion, Fe3+
For a monatomic ion the oxidation number equals the ion charge, so Fe³⁺ is +3. The 3 is read as the charge, not as three iron atoms.
Frequently asked questions
How do you find the oxidation number of an element in a compound?+
Assign the known values first (F −1, O −2, H +1, group 1 +1, group 2 +2), multiply each by its number of atoms, and solve for the unknown so the total equals the overall charge — zero for a neutral compound or the ion charge for a polyatomic ion.
Can oxidation numbers be fractions?+
Yes, an average oxidation state can be fractional when atoms of one element are in different environments. Iron in Fe3O4 averages +8/3 because it contains one Fe(II) and two Fe(III), and sulfur in tetrathionate S4O6^2- averages +2.5.
When is oxygen not −2?+
Oxygen is −1 in peroxides such as H2O2 and Na2O2, −½ in superoxides such as KO2, and positive when bonded to fluorine (+2 in OF2). It is 0 in O2 and O3.
When is hydrogen −1?+
Hydrogen is −1 in metal hydrides, where it is bonded to a less electronegative element: NaH, CaH2, LiAlH4 and NaBH4. With nonmetals such as C, N, O or Cl it is +1.
What is the difference between oxidation number and formal charge?+
Oxidation numbers assume all bonding electrons go to the more electronegative atom (fully ionic), while formal charge splits bonding electrons equally between atoms (fully covalent). They are used for redox bookkeeping and for judging Lewis structures respectively.