Question

Why does $N{H}_3$ form hydrogen bond but $P{H}_3$ does not?

Answer 1

Electronegativity values; N=3.0; P=2.1; H=2.1. Nitrogen and hydrogen have a considerable difference in their electronegativity values so they form covalent bond having polar nature. That is why H-bonding exists among $N{H}_3$ molecules.

Phosphorus and hydrogen have equal values of electronegativity, that's why covalent bond P-H is non-polar. Therefore, H-bonding does not exist among $P{H}_3$ molecules.