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QUESTION

# 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.