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The electronegativity difference between N and F is greater than that between N and H yet the dipole moment of NH3 (1.5 D) is larger than that of NF3 (0.2 D). This is because

1. in NH3 the atomic dipole and bond dipole are in the same direction whereas in NF3 these are in opposite directions
2. in NH3 as well as NF3 the atomic dipole and bond dipole are in opposite directions
3. in NH3 the atomic dipole and bond dipole are in the opposite directions whereas in NF3 these are in the same direction
4. in NH3 as well as in NF3 the atomic dipole and bond dipole are in the same direction

Correct answer: in NH3 the atomic dipole and bond dipole are in the same direction whereas in NF3 these are in opposite directions

Solution

In NH3, the lone pair on nitrogen contributes to a net dipole moment in the same direction as the bond dipoles, enhancing the overall dipole moment. In NF3, the lone pair's dipole opposes the bond dipoles, reducing the net dipole moment.

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