ACS Organic Chemistry Practice Test

In peroxides, the two oxygen atoms are bonded to each other with a single O–O bond, forming a peroxide unit O2^2-. The molecule is neutral, so the sum of oxidation states must be zero. In a representative case like hydrogen peroxide, the two hydrogens contribute +2 total, leaving -2 to come from the two oxygens. That means each oxygen carries an oxidation state of -1. This -1 per oxygen accounts for the O–O linkage and the overall neutral charge of the molecule. So the oxidation state of oxygen in peroxides is -1.