JEE Main · 2005 · 08 May 2005 · Offline · Chemical Bonding and Molecular Structure

Melting points of the alkali metal chlorides are governed by lattice energy, but with a twist for lithium. Lithium chloride would head the lattice-energy list on size alone, yet the very small, strongly polarising $\mathrm{Li}{}^{+}$ gives $\mathrm{LiCl}$ appreciable covalent character (Fajans' rules), which lowers its melting point. Sodium chloride, by contrast, is essentially purely ionic and still has a high lattice energy, so it melts the highest, after which the values fall away through $\mathrm{KCl}$ and $\mathrm{RbCl}$ as the cations grow and the lattice energy drops.