Account for the following:
(i) Acidic character increases from HF to HI.
(ii) There is large difference between the melting and boiling points of oxygen and sulphur.
(iii) Nitrogen does not form pentahalides

Explanation

(i) Acidic character increases from HF to HI. This is because the stability of these halides decreases down the group due to decrease in bond dissociation enthalpy in the order :
`H-F gt H-Cl gt H-Br gt H-I`
Therefore the acidic character follows the order :
`H-F lt H- cl lt H- Br lt H-I`
(ii) Large difference in the melting and boiling points of oxygen and sulphur can be explained on the basis of their atomicity. Oxygen exists as diatomic molecule `(O_2)` with only weak van der Waal.s forces between them whereas sulphur exists as polyatomic molecule `(S_8)` which have stronger forces of attraction between these polyatomic molecules.
(iii) Nitrogen has the electronic configuration

There are no d-orbitals in the second shell, therefore, nitrogen cannot expand its covalency beyond + 3. Hence, pentahalide formation is not possible