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.
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