Question

Why is `BaH_2` called barium hydride and not barium dihydride?

Answer 1

Because barium is in Group II of the Periodic Table.

Explanation:

Group II metals have 2 valence electrons, and form `M^(2+)` ions. They commonly form dihydrides: `CaH_2`; `BaH_2`. The `"di"` would be superfluous.