Question

A hydride is a compound containing hydride ion, $H^{-}$. Predict two elements whose hydrides would contain incomplete octets.

Answer 1

A hydride is a compound containing hydride ion.
Examples of hydrides of incomplete octets: $LiH$, and $Be{H}_2$
here the valence shell of $L{i}^{+}$ has 0 electron and $H^{-}$ ( hydride ion) has 2 electron , hence octet is incomplete.
In the same way the $B{e}^{2+}$ has 0 electrons in valence shell , and 2 hydrides ions have collectively 4 electrons in $Be{H}_2$, hence it is also incomplete.