Given reasion:
(i) Thermal stability decreases from `H_(2)O " to " H_(2)Te`
(ii) Fluoride ion has higher hydration enthalpy than chloride ion
(iii) Nitrogen does not form pentahalide

Explanation

(i) Thermal stability of hydrides decreases from `H_(2)O` to `H_(2)Te` because as the size of atom in hydrides increases from oxygen to Tellurium, the bond becomes weaker and thus, breaks on heating easily
(ii) Fluoride ion has smaller size than chloride ion due to that more water molecule can surround the fluoride ion. Hence, `F^(-)` has higher hydration enthalpy than `Cl^(-)`.
(iii) Nitrogen does not form pentahalide because nitrogen does not have empty d-orbital to form 5-bonds.