Which of the following chloride is most unstable ?

Options

A. `BCl_3`
B. `AlCl_3`
C. `Al_2 Cl_6`
D. `TlCl_3`

Answer

To determine which chloride is the most unstable among the given options (BCl3, AlCl3, Al2Cl6, and TlCl3), we will analyze the oxidation states and stability of each compound.

### Step-by-Step Solution:

1. **Identify the Chlorides and Their Elements**:
- The chlorides in question are:
- BCl3 (Boron Trichloride)
- AlCl3 (Aluminium Trichloride)
- Al2Cl6 (Dimeric form of Aluminium Trichloride)
- TlCl3 (Thallium Trichloride)

2. **Determine the Oxidation States**:
- **BCl3**: Boron (B) is in the +3 oxidation state.
- **AlCl3**: Aluminium (Al) is also in the +3 oxidation state.
- **Al2Cl6**: Each Aluminium (Al) is in the +3 oxidation state as well, since it can be represented as two AlCl3 units.
- **TlCl3**: Thallium (Tl) is in the +3 oxidation state.

3. **Analyze Stability Based on Oxidation States**:
- **BCl3**: Boron in +3 state is stable due to its small size and ability to form covalent bonds.
- **AlCl3**: Aluminium in +3 state is stable, and it can form a dimer (Al2Cl6) which is also stable.
- **Al2Cl6**: This is a stable complex formed from two AlCl3 units, maintaining the +3 oxidation state.
- **TlCl3**: Thallium in +3 state is less stable due to the inert pair effect, where the +1 oxidation state (Tl+) is more stable than the +3 state (Tl3+). This is a key factor in determining the instability of TlCl3.

4. **Conclusion**:
- Among the chlorides, TlCl3 is the most unstable due to the inert pair effect, which makes the +3 oxidation state of Thallium less favorable compared to its +1 state.

### Final Answer:
The most unstable chloride is **TlCl3** (Thallium Trichloride).

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