In both molecules i.e., NH${}_3$ and NF${}_3$, 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 NF${}_3$

is greater than NH${}_3$. However, the net dipole moment of NH${}_3$ (1.46 D) is greater than that

of NF${}_3$ (0.24 D).

This can be explained on the basis of the directions of the dipole moments of each

individual bond in NF${}_3$ and NH${}_3$. 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 NF${}_3$ is less than that of NH${}_3$.