# What is the oxidation number of oxygen in peroxides?

Mar 2, 2017

$- 1$

#### Explanation:

Oxygen almost always has an oxidation number of $- 2$, except in peroxides.

A peroxide is a molecule which includes $- O - O -$, like hydrogen peroxide, $H O O H$.

In hydrogen peroxide, the overall charge is $0$, so all the oxidation numbers must add up to $0$. The oxidation number for hydrogen is always $+ 1$, so

$H O O H = 0 = 1 \times 2 + X \times 2$

where $X$ is the oxidation number for oxygen.

From this, we can deduce that $X = - 1$.