Q1(b). Dipole moments of ammonia and nitrogen trifluoride

Answer in brief

NH₃ has the greater dipole moment: approximately 1.47 D, compared with about 0.24 D for NF₃.

Key points

• Both molecules have three bonds and a lone pair around nitrogen, giving trigonal-pyramidal geometry; bond dipoles do not cancel by symmetry.
• In NH₃, nitrogen is more electronegative than hydrogen; the N–H bond resultant reinforces the lone-pair contribution.
• In NF₃, fluorine is more electronegative than nitrogen; the N–F bond resultant opposes the lone-pair contribution, leaving a small net dipole.
• Molecular dipole is a vector sum of the complete charge distribution; larger bond polarity does not necessarily mean larger molecular polarity.