# Question #9f01e

Sep 7, 2014

Metallic salts like those of ${\text{H"_2"O}}_{2}$ don't contain oxide ions. They contain peroxide ions.

#### Explanation:

The reason is partly historical and partly chemical.

If you follow the rules for naming covalent compounds, ${\text{H"_2"O}}_{2}$ is dihydrogen dioxide.

Historically, the prefix "per-" usually means that a compound contains an extra oxygen atom. Recall that ${\text{HClO}}_{3}$ is chloric acid and ${\text{HClO}}_{4}$ is perchloric acid.

But the more important chemical reason is that metallic salts like sodium peroxide and barium peroxide don't contain oxide ($\text{O"^"2-}$) ions. They contain peroxide ($\text{O"_2^"2-}$) ions.

Compounds that are formed by combining an ${\text{H}}^{+}$ ion with an anion can have two names.

Thus, pure $\text{HCl}$ is hydrogen chloride, and the aqueous solution is hydrochloric acid.

Pure ${\text{H"_2"SO}}_{4}$ used to be called hydrogen sulfate, but it is now almost always called sulfuric acid.

In the same way, ${\text{H"_2"O}}_{2}$ was called hydrogen peroxide, and the name has stuck.