Arrange the following in the increasing order of melting point.

(i) NH₃

(ii) PH₃

(iii) AsH₃

(iv) SbH₃

PH₃ < AsH₃ < SbH₃ < NH₃

Because of intermolecular H-bonding, NH₃ has the highest melting point. The other hydrides of this group do not form H-bonds and hence their melting points are lower than that of NH₃. However as we move down the group from PH₃ to SbH₃, the molecular size increases and hence the melting point increases regularly due to a corresponding increase in their van der Waals dorces of attraction. Thus the melting points of the hydrides of group 15 elements follow the sequence:  PH₃ < AsH₃ < SbH₃ < NH₃