Account for the following:
(i) Acidic character increases from HF to HI.
(ii) There is a large difference between the melting and boiling points of oxygen and sulphur.
(iii) Nitrogen does not form pentahalide.
(i) The acidic strength of the hydrohalic acids increases from HF to HI because the stability of the acids decreases from HF to HI on account of decrease in bond dissociation enthalpy of H-X bond from HF to HI.
(ii) The oxygen exists as a diatomic molecule, O2, while sulphur exists as a polyatomic molecule, S8. Hence, there is a large difference between the melting point and the boiling point of oxygen and sulphur.
(iii) Group 15 elements form pentahalides when they have empty d-orbitals, which are can be used for forming coordinate bonds. Since nitrogen does not have d-orbitals, it cannot form petahalides.
