ACS Chemistry Practice Exam 2025 – The All-in-One Guide to Exam Success!

In peroxides, the oxidation number of oxygen is -1. This is a distinctive feature of peroxides that sets them apart from other compounds containing oxygen. Typically, oxygen has an oxidation state of -2 in most of its compounds. However, in peroxides, such as hydrogen peroxide (H2O2) or barium peroxide (BaO2), each oxygen atom is bonded to another oxygen atom, leading to a different electron distribution.

In the peroxide bond (O-O), the bonding provides a situation where the oxidation number of each oxygen atom is adjusted to -1 to maintain the charge balance of the molecule. For hydrogen peroxide, the formula can be analyzed as follows: each hydrogen contributes +1, and the two oxygen atoms share a -2 total. This confirms that each oxygen in the peroxide has an oxidation state of -1.

This understanding is crucial for recognizing how oxygen behaves in various chemical contexts and helps in predicting the properties and reactivity of peroxides compared to other oxygen-containing compounds.