Hydrogen Bonding Explains Boiling Point Difference

Steps:

• Compare molecular structures: NH3 has N-H bonds with N's high electronegativity; PH3 has P-H bonds with P's lower electronegativity.
• Assess intermolecular forces: Boiling points rise with stronger forces like hydrogen bonding over dipole-dipole.
• Evaluate NH3: N-H enables hydrogen bonding between lone pair on N and H on another molecule.
• Evaluate PH3: Lacks hydrogen bonding due to P's size and electronegativity; only weak dipole-dipole forces.

Why B is correct:

• Hydrogen bonding, a strong dipole-dipole interaction where H bonds to N/O/F, significantly raises NH3's boiling point (-33°C) versus PH3's (-88°C) per intermolecular force strength.

Why the others are wrong:

• A: Both molecules are polar due to electronegativity differences, but PH3's polarity is weaker, not absent.
• C: Bond strength influences molecular stability, not intermolecular forces determining boiling points.
• D: PH3 has a lone pair on P, but it's in a larger orbital, reducing its role in strong attractions.

Final answer: B