The dipole moment of `NF_3` is much less as compared to that of `NH_3` because

Options

A. 1. the size of N atom is much less than that of H atom
B. 2. F atom is more electronegative than N atom, whereas H atom is less electronegative than N atom
C. 3. unshared electron pair is not present in` NF_3`
D. 4. number of lone pairs in `NF_3` is much greater than in `NH_3`.

Explanation

To understand why the dipole moment of NF₃ is much less compared to that of NH₃, we can analyze the factors affecting dipole moments in these molecules step by step.

### Step-by-Step Solution:

1. **Understanding Dipole Moment**:
- The dipole moment (μ) is a measure of the separation of positive and negative charges in a molecule. It depends on the electronegativity of the atoms involved and the geometry of the molecule.

2. **Electronegativity Comparison**:
- In NH₃ (ammonia), nitrogen (N) is more electronegative than hydrogen (H). This means that the N-H bonds have a significant dipole moment directed towards nitrogen.
- In NF₃ (nitrogen trifluoride), fluorine (F) is more electronegative than nitrogen. Thus, the N-F bonds have a dipole moment directed towards fluorine.

3. **Molecular Geometry**:
- Both NH₃ and NF₃ have a trigonal pyramidal shape due to the presence of a lone pair on nitrogen. However, the lone pair in NH₃ contributes to the overall dipole moment in the same direction as the N-H bonds.
- In NF₃, the lone pair is directed opposite to the N-F bonds, which reduces the overall dipole moment.

4. **Resultant Dipole Moment**:
- In NH₃, the dipole moments of the N-H bonds add up with the contribution from the lone pair, resulting in a higher dipole moment (approximately 1.46 Debye).
- In NF₃, the dipole moments of the N-F bonds are opposed by the lone pair’s contribution, resulting in a lower dipole moment (approximately 0.24 Debye).

5. **Conclusion**:
- The dipole moment of NF₃ is much less than that of NH₃ primarily because the electronegativity of fluorine is greater than that of nitrogen, and the lone pair in NF₃ opposes the dipole moments of the N-F bonds, whereas in NH₃, the lone pair reinforces the dipole moment of the N-H bonds.

### Final Answer:
The dipole moment of NF₃ is much less compared to that of NH₃ because fluorine is more electronegative than nitrogen, and the lone pair in NF₃ opposes the dipole moments of the N-F bonds, whereas in NH₃, the lone pair supports the dipole moment of the N-H bonds.

---