Question

Why is the boiling point of `NH_3` higher than the boiling point of `PH_3` and `AsH_3`?

Answer 1

Because the degree and magnitude of intermolecular bonding between ammonia molecules is much greater than that between phosphine, and arsine molecules.

Explanation:

Ammonia, boiling point `-33` `""^@C`, has demonstrably greater intermolecular bonding than that of phosphine, boiling point `-87.7` `""^@C`, or that of arsine, boiling point `-62.5` `""^@C`. The difference in intermolecular bonding may be attributed to the degree of hydrogen bonding in each solvent. In ammonia, hydrogen is bound to a strongly electronegative element in nitrogen, and the resultant hydrogen bonding between molecules constitutes a potent intermolecular force.

On the other hand, phosphorus and arsenic are much less electronegative compared to nitrogen, and hydrogen bonding does not operate in their hydrides as an intermolecular force.