Question

Based on lattice energy and other considerations which one of the following alkali metal chloride has the highest melting point ?

• A. $KCl$
• B. $RhCl$
• C. $LiCl$
• D. $NaCl$

Answer 1

The correct option is D $NaCl$

Comparison of melting points of ionic compounds is generally done by considering the following two factors:

1. Charge of the cation/anion : More the charge of cation or anion, stronger will be the forces of attraction between the ions and higher will be the melting point.
2. Ionic radii: More the distance between ions, lesser will be the strength of the bond giving rise to lesser melting point.

Going by the above rules, the order should have been:
LiCl > NaCl > KC l> RbCl
(Since charges of the ions are same for each molecule and cationic radius increases down the group.)
But $LiCl$, due to excessive polarization, exhibits high covalent character and is placed last in the order. Hence, the new order would be:
NaCl > KCl > RbCl > LiCl
The other ions, being similar in size to chloride ions, do not undergo much polarization.