Although fluorine is much more electronegative than hydrogen yet the dipole moment of NF3(0.24D) is much lower than that of NH3(1.46D). Explain.

Explanation

Both N is `NH_3` and Bi in `BiH_3` have a lone pair of electrons on the central atom and hence should behave as Levis bases. But `NH_3` is much more basic than `BiH_3`. This can be explained on the basis of electron density on the central atom. Since the atomic size of N (70 pm) is much smaller than that of Bi (148 pm), therefore, electron density on the N-atom is much more than that on Bi-atom. Consequently, the tendency of N in `NH_3` to donate its pair of electrons is much higher than that of Bi in `BH_3`. Thus, `NH_3` is much more basic than `BiH_3`.