Chemistry - CHEMICAL BONDING Question with Solution | TestHub

Question Explanation: Explaining the difference in physical states and melting points.

Concept: Ionic vs Covalent Character (Fajans' Rule).

Solution: ${\mathrm{AlF}}_3$ is a predominantly ionic compound that forms a high-energy giant 3D ionic lattice, leading to a very high melting point. In contrast, ${\mathrm{AlCl}}_3$ has significant covalent character due to the higher polarizability of the larger ${\mathrm{Cl}}^{-}$ion. This causes ${\mathrm{AlCl}}_3$ to exist as discrete dimeric molecules $\left({\mathrm{Al}}_2{\mathrm{Cl}}_6\right)$ held by weak van der Waals forces, making it sublime at low temperatures.

${\mathbf{AlF}}_3$ : Giant ionic structure $\to$ High M.P. $\left(1291^{\circ }\mathrm{C}\right)$.

AlCl₃: Covalent dimer (Al₂Cl₆ )→ Low sublimation point (178°C).

Reason: Smaller, less polarizable F^- vs larger, polarizable Cl^- .

Final Answer: AlF₃ being a giant ionic lattice while AlCl₃ exists as discrete dimeric molecules (Al₂Cl₆ ).