`AlCl_(3)` is an electron-deficient compound but `AlF_(3)` is not, due to

Options

A. Atomic size of `F` is smaller than `Cl`, which makes `AlF_(3)` more covalent.
B. `AlCl_(3)` is a covalent compound compound while `AlF_(3)` is an ionic compound.
C. `Al` in `AlCl_(3)` is `sp^(3)` hybridised but in `AlF_(3), Al` is `sp^(3)` hybridised.
D. `AlCl_(3)` exists as a dimer but `AlF_(3)` does not.

Answer

To understand why \( \text{AlCl}_3 \) is an electron-deficient compound while \( \text{AlF}_3 \) is not, we can analyze the nature of the bonding in both compounds using Fajans' rules and the concept of covalent character in ionic compounds.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- We are comparing \( \text{AlCl}_3 \) (aluminum chloride) and \( \text{AlF}_3 \) (aluminum fluoride).

2. **Understanding Electron Deficiency**:
- An electron-deficient compound is one that does not have a complete octet around the central atom. In this case, we need to determine if aluminum in these compounds has a complete octet.

3. **Analyze the Bonding**:
- Both \( \text{AlCl}_3 \) and \( \text{AlF}_3 \) are primarily ionic compounds, but they exhibit different degrees of covalent character.

4. **Fajans' Rules**:
- According to Fajans' rules, the covalent character of an ionic bond increases with:
- The charge on the cation (aluminum in this case).
- The smaller size of the anion (chlorine vs. fluorine).
- The larger the anion, the more polarizable it is, which increases covalent character.

5. **Compare Anions**:
- Chlorine (\( \text{Cl}^- \)) is larger than fluorine (\( \text{F}^- \)).
- Due to the larger size of \( \text{Cl}^- \), it is more polarizable compared to \( \text{F}^- \).

6. **Covalent Character**:
- \( \text{AlCl}_3 \) has a higher covalent character because the larger \( \text{Cl}^- \) anion allows for more sharing of electrons with aluminum.
- In contrast, \( \text{AlF}_3 \) has a lower covalent character due to the smaller size of \( \text{F}^- \), which leads to a more ionic character.

7. **Electron Sharing vs. Transfer**:
- In \( \text{AlCl}_3 \), the bonding involves significant sharing of electrons, leading to electron deficiency in aluminum.
- In \( \text{AlF}_3 \), the bonding is more ionic, where fluorine effectively takes the electrons from aluminum, allowing aluminum to achieve a stable electron configuration.

8. **Conclusion**:
- Therefore, \( \text{AlCl}_3 \) is electron deficient due to its covalent character and the nature of the bonding, while \( \text{AlF}_3 \) is not, as it has a more ionic character.

### Final Answer:
\( \text{AlCl}_3 \) is an electron-deficient compound because it has a higher covalent character due to the larger size and polarizability of the \( \text{Cl}^- \) anion compared to the \( \text{F}^- \) anion in \( \text{AlF}_3 \).

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