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 which are dificient in hydrogen and in which the ratio of the metal to hydrogen in fractional are called non-stoichiometric hydrides. Furthermore, even this fractional ratio is not fixed but varies with the temperature and the pressure . This type of hydrides are formed by d-and f-block elements. In these hydrides , the hydrogen atoms occupy holes in the metal lattice. Usually some holes always remain unoccupied and hence these metals form non-stoichiometric hydrides.
Since alkali metals are highly reducing, they lone electron to the H atom , thereby forming `H^(-)` ions. In other words, alkali metal hydrides are ionic in which `H^(-)` ions occupy holes in the lattice . Since a hydride ion is formed by complete transfer of an electron, therefore, the ratio of metal to hydrogen is always fixed and hence alkali metals form only stoichiometric hydrides. In other words, alkali metal do not form non-stoichiometric hydrides.