Which of the following is the correct prediction about observed B - F bond length in `BF_3` molecule ?

Options

A. B - F bond length in `BF_3` is found to be less than theoretical value because the electronegativity values of B (2.04) and F (4.0) suggest the bond length to be ionic and hence , the attraction between oppositely charged ions must decrease the bond length.
B. `BF_3 and [BF_4]^(-)` have equal B - F bond length .
C. The decrease in the B - F bond length in `BF_3` is due to delocalised `ppi - ppi` bonding between vacant 2p orbital of B and filled 2p orbital of F
D. The correct B - X bond length order is `B - F gtBClgtB-BrgtB-I`

Answer

To determine the observed B-F bond length in the BF3 molecule, we need to analyze the molecular structure and bonding characteristics of BF3.

### Step-by-Step Solution:

1. **Understanding the Structure of BF3**:
- BF3 (Boron Trifluoride) consists of one boron atom and three fluorine atoms. Boron is in the center, and the three fluorine atoms are arranged around it in a trigonal planar geometry.

2. **Electronic Configuration**:
- Boron has an atomic number of 5, with an electronic configuration of 1s² 2s² 2p¹. This means boron has one electron in its 2p orbital that can participate in bonding.
- Fluorine has an atomic number of 9, with an electronic configuration of 1s² 2s² 2p⁵. Each fluorine atom has three lone pairs and one unpaired electron in its 2p orbital.

3. **Bonding in BF3**:
- In BF3, each fluorine atom forms a single bond with boron by sharing its unpaired electron. However, due to the presence of lone pairs on fluorine, there is a possibility of back-donation of electron density from the filled p orbitals of fluorine to the empty p orbital of boron.

4. **Back Bonding**:
- This back-donation creates a partial double bond character in the B-F bonds. The presence of this partial double bond character results in a stronger bond compared to a typical single bond.

5. **Effect on Bond Length**:
- The presence of partial double bond character due to back bonding causes the B-F bond length to be shorter than what would be expected for a typical single bond. Therefore, the observed B-F bond length in BF3 is less than the theoretical value for a single B-F bond.

6. **Conclusion**:
- The correct prediction about the observed B-F bond length in BF3 is that it is shorter than expected due to the presence of partial double bond character from pπ-pπ back bonding.