Key Idea : Boiling point increases with the increase in attractive forces between the molecules in liquid state. (a) NH3→ Inspite of small size of N -atoms. It has strong H-bonds due to high electronegativity of nitrogen. Thus has high boiling point. (b) PH3→ Due to the small size of P -atoms, it has least van der Walls force and will not form intermolecular H-bonds. (c) AsH3→ It has larger As-atoms which provides more van der Walls force. (d) BF3⋅PF5→ It has also larger molecular size than that of PH3 and AsH3, thus has more van der Walls force. (e) BiH3→ It has larger size than that of PH3, thus offers more van der Walls force. Thus PH3 has least boiling point.