Q.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.

Meaning of non-stoichiometric hydrides: These are hydrides (mainly formed by transition metals — d-block and f-block/lanthanoids) in which hydrogen occupies interstitial sites within the metal's crystal lattice, and the ratio of metal atoms to hydrogen atoms is not a simple, fixed whole number. The hydrogen content can vary over a range depending on preparation conditions. Examples: $LaH_{2.87}$, $TiH_{1.5-1.8}$, $VH_{0.56}$, $PdH_{0.6-0.8}$. These retain the metallic character (conductivity, lustre) of the parent metal.

Do alkali metals form them? No. Alkali metals (Group 1: Li, Na, K, Rb, Cs) react with dihydrogen to form ionic (saline) hydrides, in which hydrogen exists as the discrete hydride ion H⁻ and the metal as M⁺, giving a hydride of well-defined, fixed 1:1 stoichiometry — e.g., NaH, KH, LiH. …

Define non-stoichiometric hydrides by their variable M:H ratio and interstitial nature, identify them as a transition-metal phenomenon, and contrast with the fixed …

• Saying alkali metals DO form non-stoichiometric hydrides — this is incorrect; they form well-defined ionic MH compounds. …