An ionic hydride of an alkali metal has significant covalent character and is almost unreactive towards oxygen and chlorine. This is used in the synthesis of other useful hydrides. Write the formula of this hydride. Write its reaction with ${\mathrm{Al}}_{2}{\mathrm{Cl}}_{6}$.

It is LiH because it has significant covalent character due to the smalles alkali metal, Li. LiH is very stable. It is almost unreactive towards oxygen and chlorine.
It reacts with ${\mathrm{Al}}_{2}{\mathrm{Cl}}_{6}$ to form lithium aluminium hydride.
$8\mathrm{LiH}+{\mathrm{Al}}_{2}{\mathrm{Cl}}_{6}\to 2{\mathrm{LiAlH}}_{4}+6\mathrm{LiCl}$