# Oxidation Number Calculator - Oxidation States, Redox Agents, Electrons

> Oxidation numbers for any formula or ion with step-by-step rules, plus redox equation analysis: oxidizing and reducing agents and electrons transferred.

- URL: https://qilingtools.com/en/math/oxidation-number
- Category: Chemistry Tools (https://qilingtools.com/en/chemistry)
- Tags: [Chemistry](https://qilingtools.com/en/tags/chemistry)
- Last updated: 2026-10-26
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

An oxidation number is the charge an atom would carry if every shared electron pair were given to the more electronegative atom. Oxidation numbers are assigned by a fixed order of rules: elements are 0; fluorine is −1; group 1 metals are +1, group 2 +2 and aluminium +3; hydrogen is +1, except −1 in metal hydrides such as NaH and CaH₂; oxygen is −2, except −1 in peroxides (H₂O₂, Na₂O₂), −½ in superoxides (KO₂) and +2 in OF₂; Cl, Br and I are −1 unless bonded to oxygen; whatever is left is found from the rule that the oxidation numbers add up to the overall charge. In KMnO₄, (+1) + x + 4(−2) = 0, so Mn is +7; Cr in K₂Cr₂O₇ is +6, N in NH₄⁺ is −3 and S in S₂O₃²⁻ is +2. When atoms of one element sit in different environments the result is an average and can be a fraction: Fe in Fe₃O₄ is +8/3 and S in S₄O₆²⁻ is +2.5. This calculator applies the rules one by one, explains each assignment and shows the algebra; when two or more unknown elements remain (KSCN, K₄[Fe(CN)₆]) it searches common oxidation states for a combination that satisfies the charge, giving the negative value to the more electronegative element. In equation mode it labels every element in every species, decides whether the reaction is redox, names the elements oxidized and reduced, the oxidizing and reducing agents and the oxidation and reduction products, and — after balancing automatically — lists the electrons lost and gained and the total electrons transferred.

## How to use

1. Choose Formula / ion and type something like KMnO4, Fe3O4 or S2O3^2-, or click an example
2. Read each element's oxidation number, the rule behind it and the worked algebra
3. Switch to Equation, enter a reaction and read the oxidizing and reducing agents, products and electrons transferred

## Features

- Applies textbook rule priority and explains every assignment
- Handles ions, complexes, hydrates and nested brackets
- Recognises peroxides, superoxides, metal hydrides and OF₂
- Fractional average oxidation states shown as fraction and decimal (Fe₃O₄ +8/3)
- Solves compounds with several unknown elements via electronegativity and common states
- Equation mode colours rising (red) and falling (blue) oxidation numbers
- Identifies oxidizing/reducing agents and products, including disproportionation and comproportionation
- Electron bookkeeping after automatic balancing: electrons lost and gained and total transferred

## FAQ

### What are the rules for assigning oxidation numbers?

Free elements are 0; monatomic ions equal their charge; F is always −1; group 1 is +1, group 2 is +2, Al is +3; H is +1 (−1 with metals); O is −2 (−1 in peroxides, −½ in superoxides, +2 with F); halogens are −1 unless combined with oxygen or a more electronegative halogen; the sum equals the charge of the species. The calculator applies them in that order.

### What is the oxidation number of Mn in KMnO₄?

+7. Potassium is +1 and each oxygen is −2, so 1 + x + 4(−2) = 0 and x = +7. In the permanganate ion MnO₄⁻ the sum is −1 instead: x + 4(−2) = −1, again x = +7.

### Can an oxidation number be a fraction?

Yes, as an average. Fe₃O₄ is FeO·Fe₂O₃ — one Fe²⁺ and two Fe³⁺ — so the average is +8/3. In the tetrathionate ion S₄O₆²⁻ the two end sulfur atoms are +5 and the two bridging ones are 0, averaging +2.5. Charge balance alone can only give the average; the individual values need the structure.

### How do I identify the oxidizing and reducing agents?

The element whose oxidation number goes up is oxidized, and the reactant that contains it is the reducing agent; the element whose oxidation number goes down is reduced, and its reactant is the oxidizing agent. In 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O, Cu goes 0 → +2 (reducing agent) and N goes +5 → +2 (HNO₃ is the oxidizing agent), with 6 electrons transferred.

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