Why isn't ##H_2O_2## (hydrogen peroxide) called dihydrogen dioxide?

Metallic salts like those of ##"H"_2"O"_2## don't contain oxide ions. They contain peroxide ions.

The reason is partly historical and partly chemical.

If you follow the rules for naming , ##"H"_2"O"_2## is dihydrogen dioxide.

Historically, the prefix "per-" usually means that a compound contains an extra oxygen atom. Recall that ##"HClO"_3## is chloric acid and ##"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 (##"O"^"2-"##) ions. They contain peroxide (##"O"_2^"2-"##) ions.

that are formed by combining an ##"H"^+## ion with an anion can have two names.

Thus, pure ##"HCl"## is hydrogen chloride, and the aqueous solution is hydrochloric acid.

Pure ##"H"_2"SO"_4## used to be called hydrogen sulfate, but it is now almost always called sulfuric acid.

In the same way, ##"H"_2"O"_2## was called hydrogen peroxide, and the name has stuck.