Name the class of hydrides to which `H_(2)O_(2),B_(2)H_(6),NaH` and `LaH_(3)` belong. What is understood by 'hydrogen gap'?

Explanation

`H_(2)O` and `B_(2)H_(6)` are molecular hydrides or covalent hydrides. `B_(2)H_(6)` is in electron deficient hydride. `NaH` is saline hydride or salt-like hydride. They produce `H^(o+)` ion on electrolysis. `LaH_(3)` is metallic hydride.
'Hydride gap': The metals of group `7,8` and `9` do not from hydrides. in cact, the region of the periodic table form group `7` to group `9` is referred to as the 'hydrid gap'