Question

How are metallic hydrides formed?

Answer 1

Hydrogen gas when passed over alkali metals or alkali earth metals forms metallic hydrides.

As for examples:

$$\begin{gathered} 2\mathrm{Na}+{\mathrm{H}}_2\to \underset{\mathrm{Sodium}\mathrm{hydride}}{2\mathrm{NaH}} \\ 2\mathrm{K}+{\mathrm{H}}_2\to \underset{\mathrm{Potassium}\mathrm{hydride}}{2\mathrm{KH}} \\ \mathrm{Ca}+{\mathrm{H}}_2\to \underset{\mathrm{Calcium}\mathrm{hydride}}{{\mathrm{CaH}}_2} \\ \mathrm{Mg}+{\mathrm{H}}_2\to \underset{\mathrm{Magnesium}\mathrm{hydride}}{{\mathrm{MgH}}_2} \end{gathered}$$