What it does
An oxidation number is the charge an atom would have if every bond in the compound were fully ionic. Following them tells you which atoms are oxidised (number goes up, electrons lost) and which are reduced (number goes down, electrons gained). This page does two jobs.
- Oxidation numbers. Type a formula, with a charge for an ion (SO4^2-, NH4+, Fe^3+). The page writes each element’s oxidation number above its symbol, lists the rule that gave it, and shows the sum that must equal the overall charge. The rules are applied in a fixed order: free elements are 0; simple ions equal their charge; fluorine is −1; group 1 metals +1, group 2 +2 and aluminium +3; hydrogen +1 (but −1 in metal hydrides); oxygen −2 (except peroxides, superoxides and OF₂); and whatever is left is worked out from the sum.
- Balance a redox reaction. Pick a reducing agent (it is oxidised) and an oxidising agent (it is reduced), and choose acidic or basic solution. Each half-reaction is balanced in four steps: the main element, then oxygen with H₂O, hydrogen with H⁺, and charge with electrons. In basic solution OH⁻ is added to both sides to remove H⁺. The halves are multiplied so the electrons match, added, and tidied up, and the page checks that atoms and charge balance.
It follows Class 11 Chemistry (redox reactions: oxidation number, the rules for assigning it, types of redox reactions, balancing redox equations by the oxidation number and half-reaction methods). Write formulas with the formula writer, balance other equations in the equation balancer, and turn redox reactions into batteries in the galvanic cell builder.
How to use it
- Type a formula or tap an example. Read the numbers, the rule for each and the check.
- Open The rules to see them all; the ones used are in bold.
- In the second card, choose the two half-reactions and the medium.
- Follow the numbered steps for each half, then read the final equation.
Worked example: potassium dichromate
Type K2Cr2O7. Potassium is +1 (group 1) and oxygen −2. The compound is neutral, so
2 × (+1) + 2 × x + 7 × (−2) = 0, which gives 2x = 12 and x = +6.
The page shows K +1, Cr +6, O −2.
Now balance iron(II) with permanganate in acid. Oxidation: Fe²⁺ → Fe³⁺ + e⁻ (iron goes from +2 to +3). Reduction: MnO₄⁻ → Mn²⁺; add 4H₂O on the right for the oxygen, 8H⁺ on the left for the hydrogen, and 5e⁻ on the left for the charge: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (manganese from +7 to +2). Multiply the iron half by 5 and add:
5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O
Both sides have charge +17.
Tips
- A fraction, such as +8/3 for iron in Fe₃O₄, is an average over atoms in different states.
- Where two or more elements have no fixed rule (such as ICl or PCl₅), the most electronegative gets its usual negative value and the page says the result is an estimate.
- In basic solution the final equation has OH⁻ instead of H⁺. The electrons lost and gained are the same in either medium.
FAQ
Is an oxidation number a real charge? Only for simple ions. In molecules it is bookkeeping that helps you follow electrons.
Is anything saved? No. Everything runs in your browser and nothing is stored or sent.
Is this an official textbook tool? No. It is an independent practice page that follows the same school topics.
Where the numbers come from
- Oxidation number rules and the half-reaction (ion-electron) method: general chemistry facts as set out by IUPAC and in standard textbooks, written fresh for fibub.
- Pauling electronegativities (used only to order elements that have no fixed rule) from fibub's periodic table data.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.