Concept:According to Fajans' Rules, the covalent character of an ionic compound increases with a high positive charge and small size of the cation (high polarizing power).
Formula:$$ \text{Polarizing Power} \propto \frac{\text{Charge}}{\text{Radius}} $$
Solution:- Aluminum in \( \text{AlCl}_3 \) has a very high charge (\( 3^+ \)) and a small ionic radius compared to Na, K, and Mg.
- This gives \( \text{Al}^{3+} \) extreme polarizing power, allowing it to strongly distort (polarize) the electron cloud of the chloride anions.
- This heavy distortion pulls the electrons into the space between the nuclei, creating substantial electron sharing (covalent character). Therefore, \( \text{AlCl}_3 \) exists predominantly as a covalent molecule.
Why other options are incorrect:- Option A, Option C, Option D: These metals (Na, Mg, K) have lower charges (\( 1^+, 2^+ \)) and larger radii, leading to weak polarizing power and predominantly ionic bonds.
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