O22- ion name

The ion written O₂²⁻ is called the peroxide ion. It contains two oxygen atoms joined together and has an overall charge of 2−, giving each oxygen an average oxidation state of −1.

How it works

This differs from oxide, O²⁻, which is a single oxygen atom with a 2− charge, and superoxide, O₂⁻, which is a diatomic ion with a 1− charge. The subscript and charge both matter when reading a chemical formula.

Example or application

Hydrogen peroxide, H₂O₂, contains the peroxide group. In ionic compounds such as sodium peroxide, Na₂O₂, two sodium ions balance the charge of one peroxide ion.

Scope and limits

Formula notation can be easy to misread. Use parentheses and charge conventions carefully, and do not confuse an ion’s formal charge with the oxidation state of every atom in other compounds.

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