question_answer 16)

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

Answer:

The hydrides in which the ratio of the metal and hydrogen is found to be fractional are called non-stoichiometric hydrides. It is also observed that this fractional ratio is not fixed but varies with the temperature and pressure. Such hydrides are formed by \[\text{d-}\]and \[\text{f}\,\text{-}\,\text{block}\] elements. In these hydrides, hydrogen atoms occupy holes in the metal lattice. Usually all the holes are not occupied, i.e., some holes always remain vacant and thus, these compounds are non-stoichiometric. Alkali metals do not form non-stoichiometric hydrides as each sodium atom loses its valency electron which is accepted by hydrogen atom to form \[{{H}^{-}}\] ion. In this way, an ionic compound, \[N{{a}^{+}}{{H}^{-}}\], is formed.