What do you understand by the term "non-stoichiometric hydrides"? Do you expect this type of the hydrides to be formed by alkali metals? Justify your answer.

Explanation

Hydrides of d and f-block elements which are deficient in hydrogen and in which the ratio of the metal to hydrogen is fractional are called non-stoichiometric hydrides.
The alkali metals, being highly reducing in nature, transfer their lone-pair of electron to the H-atom forming `H^(-)`ions. Since a hydride ion `H^(-)` is formed by complete transfer of an electron, the ratio of metal of hydrogen is always fixed in these hydrides and their compositions correspond to simple whole number ratio. Hence, the alkali metals form only stoichiometric hydrides.