Question

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

Answer 1

The dipole moment of NH3 is higher in comparison to NF3. 

REASON 

The F i.e. fluorine is more electronegative than N and pulls the electrons . As the number NF bonds is 3 , the 3 F atoms pull the electrons. The resulting electronegative forces decrease the dipole moment. 

NF3 DIPOLE MOMENT 0.24D

The NH3 molecule having 3 Hydrogen molecules that form bond with N do not have electronegativity to over come the pull of N molecule (unlike the NF3 ) which results in higher dipole moment . 

NH3 DIPOLE MOMENT 1.24D.

Answer 2

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.