Question

`NH_3` has a much higher boiling point than `PH_3` does. Why?

Answer 1

Consider `Delta""EN"`, where `"EN"` is the electronegativity of phosphorus versus nitrogen.

Explanation:

Pauling electronegativities of hydrogen is `2.1`, versus phosphorus, `2.2`, versus nitrogen, `3.1`.

`Delta""EN"` is much greater for the amine. The electronegativity difference allows for effective hydrogen bonding between ammonia molecules, and thus amounts to a very significant intermolecular force. The intermolecular force is manifested by the normal boiling points of ammonia, `-33` `""^@C`, versus phosphine, `-88` `""^@C`.

Ammonia is in fact a very water like solvent given its propensity to hydrogen bond, and is very soluble in water. Contrast this with the low solubility of phosphine in water. Mind you, the smell of ammonia will literally bring tears to your eyes.

Note here that I speak of the intermolecular force, the force between individual molecules, and not the intramolecular force, the strength of `N-H` and `P-H` bonds.