What is an oxidation number?

An oxidation number (also called oxidation state) is a number assigned to an element in a compound that represents the hypothetical charge the atom would have if all bonds were fully ionic. It helps track electron transfer in redox reactions.

What rules are used to assign oxidation numbers?

The rules applied in order are: (1) Free elements = 0. (2) Monatomic ions = their charge. (3) Fluorine is always -1. (4) Oxygen is -2 (except in peroxides where it is -1 or in OF2 where it is +2). (5) Hydrogen is +1 with nonmetals and -1 with metals. (6) The sum of all oxidation numbers equals the overall charge of the species.

Why does oxygen sometimes have a different oxidation number?

Oxygen is normally -2, but in peroxides (like H2O2) it is -1 because the O-O bond shares electrons differently. In OF2, fluorine's higher electronegativity forces oxygen to be +2.

Can an element have multiple oxidation states?

Yes. Many transition metals (Fe, Cu, Mn, Cr) and nonmetals (S, N, Cl) have multiple possible oxidation states depending on the compound. The calculator uses the charge-balance rule to determine the correct state for each specific formula.

What does it mean if an oxidation number is positive or negative?

A positive oxidation number means the atom has lost or donated electrons (oxidized). A negative oxidation number means the atom has gained electrons (reduced). The more positive the number, the greater the degree of oxidation.

Oxidation Number Calculator

Name: Oxidation Number Calculator
Availability: InStock
Author: Nham Vu

Enter a chemical compound or formula and get the oxidation number of each element with a step-by-step rules explanation.

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Common Compounds

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Use element symbols with subscript numbers: H2O, NaCl, Fe2O3, K2Cr2O7

Overall charge (0 for neutral compounds)

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Oxidation numbers and step-by-step rules appear here

Summary

Enter a chemical compound or formula and get the oxidation number of each element with a step-by-step rules explanation.

How it works

Select a compound from the preset list or type a formula like H2O or NaCl.
The calculator applies standard oxidation number rules in priority order.
Each element in the compound is assigned its oxidation number.
A step-by-step explanation shows which rule was applied for each assignment.
Verify that oxidation numbers sum to the correct charge for the compound.

Use cases

Check oxidation states for homework and exam prep.
Learn oxidation number rules with guided explanations.
Verify redox reaction assignments before balancing equations.
Quickly look up oxidation states in common ionic and covalent compounds.
Understand why certain elements have variable oxidation states.
Identify the oxidizing and reducing agents in a reaction.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu