Question

Which out of NH₃ and NF₃ has higher dipole moment and why?

Answer 1

In both molecules i.e., NH₃ and NF₃, the central atom (N) has a lone pair electron and there are three bond pairs. Hence, both molecules have a pyramidal shape. Since fluorine is more electronegative than hydrogen, it is expected that the net dipole moment of NF₃ is greater than NH₃. However, the net dipole moment of NH₃(1.46 D) is greater than that of NF₃ (0.24 D).

This can be explained on the basis of the directions of the dipole moments of each individual bond in NF₃ and NH₃. These directions can be shown as:

Thus, the resultant moment of the N–H bonds add up to the bond moment of the lone pair (the two being in the same direction), whereas that of the three N – F bonds partly cancels the moment of the lone pair.

Hence, the net dipole moment of NF₃ is less than that of NH₃.

Answer 2

In NH₃ and NF₃, the difference in electronegativity is nearly same but the dipole moment of NH₃ = (1.46D) For Example, NH₃ = (0.24D)

In NH3, the dipole moments of the three N—H bonds are in the same direction as the lone pair of electron. But in NF₃, the dipole moments of the three N—F bonds are in the direction opposite to that of the lone pair. Therefore, the resultant dipole moment in NH₃ is more than in NF₃.