Question 23: Which out of NH3 and NF3 has higher dipole moment and why?
Answer:
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.
Since in case of NF3, fluorine is more electronegative than nitrogen, so it will attract the shared pair of electron towards itself from nitrogen. As a result dipole due to all F is in opposite direction to the resultant dipole due to N-F bonds, therefore the net dipole decreases.
In NH3, Nitrogen is more electronegative than H, so it will attract the shared pair of electrons towards itself. As a result dipole due to Nitrogen is in same direction as the resultant dipole moment due to N-H bond and hence the net dipole increases.
Hence, the net dipole moment of NF3 is less than that of NH3.
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