`AlCl_(3)` forms dimer in vapour phase but `BCl_(3)` does not because

Options

A. In Al there are vacant d orbitals in which it accommodates lone pair from chlorine atoms
B. In `BCl_(3)` there is back bonding
C. There is hyrogen bonding in between two `AlCl_(3)` molecules in vapour phase
D. None of these

Answer

To understand why \( \text{AlCl}_3 \) forms a dimer in the vapor phase while \( \text{BCl}_3 \) does not, we need to analyze the electronic configurations and the properties of the central atoms involved.

### Step-by-Step Solution:

1. **Identify the Central Atoms**:
- The central atom in \( \text{AlCl}_3 \) is Aluminum (Al), and in \( \text{BCl}_3 \) it is Boron (B).

2. **Electronic Configurations**:
- Aluminum (Al) has an atomic number of 13, and its electronic configuration is:
\[ \text{Al: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^1 \]
- Boron (B) has an atomic number of 5, and its electronic configuration is:
\[ \text{B: } 1s^2 \, 2s^2 \, 2p^1 \]

3. **Vacant d-Orbitals**:
- Aluminum, being in the third period, has vacant d-orbitals (3d) that can accommodate electron pairs. This allows \( \text{AlCl}_3 \) to form dimers by utilizing these vacant d-orbitals to bond with additional chlorine atoms.
- Boron, on the other hand, does not have any vacant d-orbitals (it only has 2s and 2p orbitals), which limits its ability to form dimers.

4. **Backbonding Consideration**:
- In \( \text{AlCl}_3 \), the presence of vacant d-orbitals allows for backbonding with the lone pairs of the chlorine atoms. This interaction stabilizes the dimer formation.
- In \( \text{BCl}_3 \), the lack of vacant d-orbitals means it cannot effectively accommodate the lone pairs from chlorine, thus it does not form dimers.

5. **Steric Hindrance**:
- Additionally, \( \text{BCl}_3 \) experiences steric hindrance due to the larger size of chlorine compared to fluorine (in \( \text{BF}_3 \)), which further prevents dimerization.

### Conclusion:
The primary reason \( \text{AlCl}_3 \) forms a dimer in the vapor phase while \( \text{BCl}_3 \) does not is due to the presence of vacant d-orbitals in aluminum that can accommodate lone pairs from chlorine, facilitating dimer formation. In contrast, boron lacks these vacant d-orbitals, preventing it from forming dimers.