Question

What is the oxidation number of oxygen in peroxides?

Answer 1

`-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, `HOOH`.

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

`HOOH = 0 = 1 xx 2 + X xx 2`

where `X` is the oxidation number for oxygen.

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