Q.Why does NH3 form hydrogen bond but PH3 does not?

Step 1: Condition for hydrogen bonding.

Hydrogen bonding requires a hydrogen atom covalently bonded to a small, highly electronegative atom (like N, O, F) that also carries a lone pair, so that a strong permanent dipole forms and can attract a lone pair on a neighbouring electronegative atom.

Step 2: Nitrogen in $NH_3$.

Nitrogen is small in size and has high electronegativity (3.0 on Pauling scale). This makes the N–H bond significantly polar, with a substantial partial negative charge concentrated on the small nitrogen atom. This is strong enough to attract an N–H hydrogen from an adjacent $NH_3$ molecule, giving intermolecular hydrogen bonding.

Step 3: Phosphorus in $PH_3$.

Phosphorus has a much larger atomic size and considerably lower electronegativity (2.1) than nitrogen. The P–H bond is therefore only weakly polar, and the partial negative charge on phosphorus is too diffuse/small to set up effective hydrogen bonding between $PH_3$ molecules. …

State the two conditions needed for hydrogen bonding (small size + high electronegativity of the atom attached to H), the …

• Do not just say "N is more electronegative than P" — size is equally essential to the hydrogen-bonding argument and must be mentioned.
• Do not confuse this with dipole moment comparisons alone; hydrogen bonding is a specific, stronger intermolecular interaction requiring both a polar bond AND a small electronegative atom …