Oxidation Number Calculator

How to Calculate Oxidation Number

The oxidation number (or oxidation state) indicates the degree of oxidation of an atom in a chemical compound. Understanding how to calculate it is a foundational skill in chemistry, especially for redox reactions.

General Rules for Oxidation States

• Free elements: Always 0 (e.g., O2, S8, Fe).
• Alkali metals (Group 1): Always +1 (e.g., Na, K).
• Alkaline earth metals (Group 2): Always +2 (e.g., Mg, Ca).
• Fluorine: Always -1.
• Hydrogen: Usually +1 (except in metal hydrides where it's -1).
• Oxygen: Usually -2 (except in peroxides like H2O2 where it's -1, or bound to Fluorine).

Using the Formula

The sum of all oxidation numbers in a neutral molecule must equal 0. For polyatomic ions, the sum must equal the net charge of the ion.

Example: Calculate the oxidation number of Cr in Cr2O7^2-

1. Let the oxidation number of Cr be x.
2. Oxygen is -2. There are 7 oxygen atoms, so 7 * (-2) = -14.
3. The net charge of the ion is -2.
4. Set up the equation: 2x + (-14) = -2
5. Solve for x: 2x = +12 ⇒ x = +6.

Our Oxidation Number Calculator automates this process for any complex chemical formula, including those with nested brackets like [Fe(CN)6]3-.