Concept: Fajan's Rules

Fajan's rules help predict the degree of covalency in an ionic compound. According to Fajan's rules, a compound is more covalent if:

1. The cation has a high charge.
2. The cation is small.
3. The anion is large.
4. The cation has a pseudo noble gas configuration (e.g., d-block elements).

In this question, we are comparing halides of the same cation, Aluminum (Al³⁺). The cation (Al³⁺) is common to all options, so we need to consider the size of the anion.

Why (D) is correct:

The anions are F^-, Cl^-, Br^-, and I^-. The size of the halide ions increases down the group:

$${\text{F}}^{-}<{\text{Cl}}^{-}<{\text{Br}}^{-}<{\text{I}}^{-}$$

According to Fajan's rules, a larger anion is more easily polarized by the cation, leading to a greater degree of covalency. Since I^- is the largest anion among the given options, AlI₃ will have the highest degree of covalent character.

Option Analysis:

• A) AlF₃: Fluoride (F^-) is the smallest halide ion, making AlF₃ the most ionic among the given options.
• B) AlCl₃: Chloride (Cl^-) is larger than F^- but smaller than Br^- and I^-. It is less covalent than AlBr₃ and AlI₃.
• C) AlBr₃: Bromide (Br^-) is larger than F^- and Cl^- but smaller than I^-. It is less covalent than AlI₃.
• D) AlI₃: Iodide (I^-) is the largest halide ion. Its electron cloud is most easily distorted (polarized) by the Al³⁺ cation, leading to the highest covalent character.

Correct Answer: (D)