Question

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

Answer 1

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 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 $NF_3$ is less than that of $NH_3$.