The `BF_3` is a planar molecule where as `NF_3` is pyramidal because :

Options

A. B-F bond is more polar than N-F bond
B. Boron atom is bigger than nitrogen atom
C. Nitrogen is more electronegative than boron
D. `BF_3` has no lone pair but `NF_3` has a lone pair of electrons.

Explanation

To understand why BF3 is a planar molecule while NF3 is pyramidal, we need to analyze the molecular structures and hybridizations of both compounds step by step.

### Step-by-Step Solution:

1. **Identify the Central Atom**:
- For BF3, the central atom is Boron (B).
- For NF3, the central atom is Nitrogen (N).

2. **Determine Valence Electrons**:
- Boron (B) has 3 valence electrons.
- Nitrogen (N) has 5 valence electrons.

3. **Count Monovalent Atoms**:
- In BF3, there are 3 fluorine (F) atoms.
- In NF3, there are also 3 fluorine (F) atoms.

4. **Calculate Hybridization**:
- The formula for hybridization is:
\[ \text{Hybridization} = \frac{\text{Valence of central atom} + \text{Number of monovalent atoms} + \text{Charge}}{2} \]

- **For BF3**:
\[ \text{Hybridization} = \frac{3 + 3 + 0}{2} = \frac{6}{2} = 3 \quad (\text{sp}^2) \]
- **For NF3**:
\[ \text{Hybridization} = \frac{5 + 3 + 0}{2} = \frac{8}{2} = 4 \quad (\text{sp}^3) \]

5. **Determine Molecular Geometry**:
- **BF3**: With sp² hybridization and no lone pairs, the molecular geometry is trigonal planar.
- **NF3**: With sp³ hybridization and one lone pair, the molecular geometry is trigonal pyramidal.

6. **Conclusion**:
- The difference in molecular shapes arises from the presence of a lone pair in NF3, which repels the bonding pairs and creates a pyramidal shape. In contrast, BF3 has no lone pairs, resulting in a planar structure.

### Final Answer:
BF3 is a planar molecule because it has no lone pairs on the central atom (Boron), whereas NF3 is pyramidal due to the presence of a lone pair on the central atom (Nitrogen).