`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 AlCl₃ forms a dimer in the vapor phase while BCl₃ does not, we can analyze the electronic configurations and bonding characteristics of both compounds step by step.

### Step 1: Analyze the Electronic Configuration
- **Aluminum (Al)** has the electronic configuration:
\[ 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^1 \]
- **Boron (B)** has the electronic configuration:
\[ 1s^2 \, 2s^2 \, 2p^1 \]

### Step 2: Identify the Valence Shells
- Aluminum is in the third period and has access to the 3s, 3p, and 3d orbitals.
- Boron is in the second period and only has access to the 2s and 2p orbitals. It does not have d orbitals available.

### Step 3: Dimer Formation in AlCl₃
- In AlCl₃, the aluminum atom can utilize its vacant 3d orbitals to accommodate the lone pairs from the chlorine atoms. This allows for the formation of a dimer (Al₂Cl₆) through coordinate covalent bonding.

### Step 4: Lack of Dimer Formation in BCl₃
- In BCl₃, there are no vacant d orbitals available for bonding with the lone pairs from chlorine. As a result, BCl₃ cannot form dimers. Instead, it exists as discrete BCl₃ molecules.

### Step 5: Evaluate the Options
- **Option A**: "In Al there are vacant d-orbitals in which it accommodates lone pair from chlorine atoms." - This is true.
- **Option B**: "In BCl₃ there is back bonding." - This is not applicable as BCl₃ does not have d orbitals for back bonding.
- **Option C**: "There is hydrogen bonding in between AlCl₃ molecules in vapor phase." - This is false, as neither molecule contains hydrogen.
- **Option D**: "None of these." - Since Option A is correct, this cannot be the answer.

### Conclusion
The correct answer is **Option A**: "In Al there are vacant d-orbitals in which it accommodates lone pair from chlorine atoms."

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