Which is the correct arrangement of the molecules based on dipole moments ?

Options

A. `BF_(3) = NH_(3) = NF_(3)`
B. `BF_(3) gt NH_(3) gt NF_(3)`
C. `BF_(3) lt NH_(3) lt NF_(3)`
D. `BF_(3) lt NF_(3) lt NH_(3)`

Answer

To determine the correct arrangement of the molecules BF3, NH3, and NF3 based on their dipole moments, we will follow these steps:

### Step 1: Draw the Lewis Structures
1. **BF3 (Boron Trifluoride)**:
- Boron (B) is the central atom.
- Three fluorine (F) atoms are bonded to boron.
- The Lewis structure shows boron with three single bonds to three fluorine atoms and no lone pairs on boron.

2. **NH3 (Ammonia)**:
- Nitrogen (N) is the central atom.
- Three hydrogen (H) atoms are bonded to nitrogen.
- The Lewis structure shows nitrogen with three single bonds to hydrogen and one lone pair on nitrogen.

3. **NF3 (Nitrogen Trifluoride)**:
- Nitrogen (N) is the central atom.
- Three fluorine (F) atoms are bonded to nitrogen.
- The Lewis structure shows nitrogen with three single bonds to three fluorine atoms and one lone pair on nitrogen.

### Step 2: Determine Electronegativity and Dipole Moments
1. **BF3**:
- Boron has a lower electronegativity than fluorine.
- The dipole moments from each B-F bond point towards fluorine.
- The symmetrical arrangement of the three B-F bonds leads to cancellation of dipole moments.
- **Net dipole moment = 0**.

2. **NH3**:
- Nitrogen has a higher electronegativity than hydrogen.
- The dipole moments from each N-H bond point towards nitrogen.
- The dipole moments add up due to the asymmetrical shape (pyramidal) of the molecule.
- **Net dipole moment > 0**.

3. **NF3**:
- Fluorine has a higher electronegativity than nitrogen.
- The dipole moments from each N-F bond point towards fluorine.
- The presence of the lone pair on nitrogen causes some cancellation of dipole moments, but not completely.
- **Net dipole moment < NH3 but > 0**.

### Step 3: Compare the Dipole Moments
- From the analysis:
- **BF3** has a dipole moment of **0**.
- **NF3** has a dipole moment that is **greater than 0** but **less than NH3**.
- **NH3** has the highest dipole moment.

### Final Arrangement Based on Dipole Moments
The correct arrangement of the molecules based on their dipole moments is:
1. NH3 (highest dipole moment)
2. NF3 (intermediate dipole moment)
3. BF3 (dipole moment = 0)

### Conclusion
The correct order based on dipole moments is:
**NH3 > NF3 > BF3**