Q.PH3 has lower boiling point than NH3. Why?

Step 1 — Intermolecular forces in $NH_3$.

Nitrogen is small and highly electronegative (3.04 on the Pauling scale). The N–H bond is polar enough, and N is small enough, that one $NH_3$ molecule's N lone pair can hydrogen-bond to the H of a neighbouring molecule:

This extensive hydrogen bonding raises the energy needed to separate molecules into the gas phase, giving $NH_3$ a comparatively high boiling point (–33 °C).

Step 2 — $PH_3$ has no hydrogen bonding.

Phosphorus is larger and far less electronegative (2.19) than N, so the P–H bond is only weakly polar and P is too big/diffuse to support hydrogen bonding. $PH_3$ molecules interact only through weak van der Waals (dispersion) forces.

Step 3 — Result. …

Identify whether hydrogen bonding is possible by checking the central atom's electronegativity and size; where it is absent, boil …

• Don't assume boiling point simply tracks molar mass — $PH_3$ is heavier than $NH_3$ yet boils lower, because intermolecular force type (H-bonding vs van der Waals) dominates here. …