Why does phosphine have a lower boiling point than ammonia?

Nitrogen atom is small so it is capable of form hydrogen bonding. So the neighboring ammonia molecules get closer due to the attraction. We need more energy to separate them than Phosphine. In Phosphine, P atom is too large to form hydrogen bond. read more

Nitrogen is sufficiently electronegative to polarise the N-H bonds in ammonia and hence is able to lead to the formation of hydrogen bonds between ammonia molecules. read more

PH3 does not exhibit hydrogen bonding and the dominant intermolecular force holding these molecules together is dispersion forces. (Dispersion forces also known as Van Der Waal Force). Hydrogen bonding takes more energy to break than dispersion forces and therefore gives ammonia the higher boiling point. read more