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The electronegativity difference between N and F is greater than that between N and H, yet the dipole moment of $NH_3$ (1.5 D) is larger than that of $NF_3$ (0.2 D). This is because :-
A
In $NH_3$ as well as in $NF_3$ the atomic dipole and bond dipole are in the same direction
B
In $NH_3$ the atomic dipole and bond dipole are in the same direction whereas in $NF_3$ these are in opposite directions
C
In $NH_3$ as well as $NF_3$ the atomic dipole and bond dipole are in opposite directions
D
In $NH_3$ the atomic dipole and bond dipole are in the opposite directions whereas in $NF_3$ these are in the same direction
Detailed Solution
Both $NH_3$ and $NF_3$ are pyramidal, with a lone pair of electrons on nitrogen.
The net dipole moment is the vector sum of the bond dipoles and the orbital (atomic) dipole due to the lone pair, which points away from the nitrogen nucleus.
In $NH_3$, nitrogen is more electronegative than hydrogen, so each N-H bond dipole points towards nitrogen. The resultant of the three bond dipoles is in the same direction as the lone pair dipole, so they add up and give a large dipole moment.
In $NF_3$, fluorine is more electronegative than nitrogen, so each N-F bond dipole points towards fluorine, away from nitrogen. The resultant of the bond dipoles is opposite to the lone pair dipole, so they partly cancel and the dipole moment is small.
Hence in $NH_3$ the atomic dipole and bond dipole are in the same direction, whereas in $NF_3$ these are in opposite directions.
The net dipole moment is the vector sum of the bond dipoles and the orbital (atomic) dipole due to the lone pair, which points away from the nitrogen nucleus.
In $NH_3$, nitrogen is more electronegative than hydrogen, so each N-H bond dipole points towards nitrogen. The resultant of the three bond dipoles is in the same direction as the lone pair dipole, so they add up and give a large dipole moment.
In $NF_3$, fluorine is more electronegative than nitrogen, so each N-F bond dipole points towards fluorine, away from nitrogen. The resultant of the bond dipoles is opposite to the lone pair dipole, so they partly cancel and the dipole moment is small.
Hence in $NH_3$ the atomic dipole and bond dipole are in the same direction, whereas in $NF_3$ these are in opposite directions.