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Although the electronegativity gap between N and F is larger than that between N and H, ammonia has a dipole moment of 1.5 D while nitrogen trifluoride has only 0.2 D. The reason is that

1. in NH₃, the atomic dipole and bond dipole point in the same direction, whereas in NF₃ they point in opposite directions
2. in both NH₃ and NF₃, the atomic dipole and bond dipole point in opposite directions
3. in NH₃ the atomic dipole and bond dipole point in opposite directions, whereas in NF₃ they point in the same direction
4. in both NH₃ and NF₃, the atomic dipole and bond dipole point in the same direction

Correct answer: in NH₃, the atomic dipole and bond dipole point in the same direction, whereas in NF₃ they point in opposite directions

Solution

In ammonia (NH₃), the dipoles from the N-H bonds add up in the same direction, resulting in a significant overall dipole moment. In contrast, in nitrogen trifluoride (NF₃), the dipoles from the N-F bonds are directed opposite to the dipole from the nitrogen atom, which cancels out much of the overall dipole moment, leading to a much smaller value.

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