# Which gas has the greater degree of intermolecular force: ammonia, NH_3(g); or phosphine, PH_3(g)?

Phosphine, $\text{b.p. } - 87.7$ ""^@C; ammonia, $\text{b.p. } - 33.3$ ""^@C;
Given the boiling points, CLEARLY, ammonia, $N {H}_{3}$, has the greatest degree of intermolecular bonding. How do we rationalize this? The obvious discriminant is hydrogen bonding. There is little electronegativity difference between phosphorus and hydrogen, and yet a large difference exists between nitrogen and hydrogen.
The $N - H$ bond is strongly polar on this account. The result is that the ammonia participates in intermolecular hydrogen bonding, which is a potent intermolecular force.