Which of the following is ionic

Options

A. `AlF_3`
B. `AlCl_3`
C. `AlBr_3`
D. `AlI_3`

Answer

To determine which of the compounds among AlF3, AlCl3, AlBr3, and AlI3 is ionic in nature, we can follow these steps:

### Step 1: Understand Ionic vs. Covalent Compounds
Ionic compounds are formed when there is a significant difference in electronegativity between the cation and anion, leading to the transfer of electrons. Covalent compounds, on the other hand, involve the sharing of electrons.

### Step 2: Apply Fajans' Rule
Fajans' Rule helps predict whether a compound will have ionic or covalent character based on several factors:
1. The size of the cation (smaller is better for ionic character)
2. The size of the anion (larger size leads to more covalent character)
3. The charge on the cation and anion (higher charge increases ionic character)
4. The polarizability of the anion (larger anions are more polarizable)

### Step 3: Analyze Each Compound
- **AlF3**: Aluminum (Al) is a small cation, and fluorine (F) is a small anion. This combination tends to favor ionic character.
- **AlCl3**: Chlorine (Cl) is larger than fluorine, which may introduce some covalent character.
- **AlBr3**: Bromine (Br) is larger than chlorine, leading to even more covalent character.
- **AlI3**: Iodine (I) is the largest anion among these, which significantly increases the covalent character.

### Step 4: Compare Ionic and Covalent Character
As we move from AlF3 to AlI3, the size of the anion increases, which increases the covalent character and decreases the ionic character:
- AlF3 has the highest ionic character due to the small size of F.
- AlCl3 has moderate ionic character.
- AlBr3 has lower ionic character.
- AlI3 has the lowest ionic character and is more covalent.

### Conclusion
Based on the analysis, the compound that is predominantly ionic is **AlF3**.

### Final Answer
**AlF3 is ionic in nature.**

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