Based on lattice energy and other considerations, which one of the following alkali metal chloride is expected to have the highest melting point?

The melting point of ionic compounds generally depends on two competing periodic trends:

1. Lattice Energy: As the size of the alkali metal cation increases down the group ($$\text{Li}^+ < \text{Na}^+ < \text{K}^+ < \text{Rb}^+$$), the interionic distance increases, which decreases the lattice energy. Lower lattice energy generally decreases the melting point. Based on this alone, we would expect $$\text{LiCl}$$ to have the highest melting point.
2. Polarization and Covalent Character (Fajan's Rules): According to Fajan's rules, a smaller cation has a higher charge density and a greater ability to polarize the electron cloud of the chloride anion. This introduces significant covalent character into the bond, which significantly lowers the melting point.

Detailed Analysis of the Halides:

• $$\text{LiCl}$$:

  Because the $$\text{Li}^+$$ ion is exceptionally small, it highly polarizes the $$\text{Cl}^-$$ ion, making $$\text{LiCl}$$ predominantly covalent. Consequently, its melting point drops unexpectedly low ($$605 \ ^\circ\text{C}$$).




• $$\text{NaCl}$$:

  The $$\text{Na}^+$$ ion is larger than $$\text{Li}^+$$, meaning it has a much lower polarizing power, and the compound retains highly ideal ionic character. At the same time, because $$\text{Na}^+$$ is smaller than $$\text{K}^+$$ and $$\text{Rb}^+$$, it maintains a relatively high lattice energy. This perfect balance between strong ionic binding and high lattice energy gives it the peak melting point ($$801 \ ^\circ\text{C}$$).




• $$\text{KCl}$$ and $$\text{RbCl}$$:

  As we move further down to $$\text{K}^+$$ and $$\text{Rb}^+$$, the compounds remain ionic, but the increasing size of the cations drastically diminishes the lattice energy. As a result, their melting points decrease progressively ($$770 \ ^\circ\text{C}$$ for $$\text{KCl}$$ and $$718 \ ^\circ\text{C}$$ for $$\text{RbCl}$$).

Conclusion:

Accounting for both the decrease in lattice energy and the introduction of covalent character via Fajan's rules, the overall melting point order for group 1 chlorides is:

$$\text{NaCl} > \text{KCl} > \text{RbCl} > \text{LiCl}$$

Therefore, sodium chloride possesses the highest melting point among the choices.

Answer: Option A — NaCl