Question

Which one of the following order is correct regarding their dipole moments?

(a) BF₃ > NF₃ > NH₃

(b) NF₃ > BF₃ > NH₃

(c) NH₃ > BF₃ > NF₃

(d) NH₃ > NF₃ > BF₃

Answer 1

Correct option is (d) NH₃ > NF₃ > BF₃

BF₃ is trigonal planar; the bond moment due to two B-F bonds cancel the bond moment due to the third B-F bond. NH₃ and NF₃ are pyramidal and hence have some dipole moments. Dipole moment in NF₃ is less than that of NH₃ because dipole moments due to N - F bonds and due to lone pair of electrons are in opposite directions due to electronegativity value (N = 3.0, F = 4.0) while in NH₃ dipole moments due to N - H bonds as well as due to lone pair of electrons is in the same direction.