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

The melting point of an ionic compound depends on two main competing factors:

1. Lattice Energy ($$U$$): This represents the strength of the electrostatic forces between ions in a crystal lattice. According to Coulomb's Law, lattice energy is inversely proportional to the interionic distance ($$r_+ + r_-$$):

   $$U \propto \frac{1}{r_+ + r_-}$$

   As you move down Group 1 from $$\text{Li}^+$$ to $$\text{Rb}^+$$, the size of the alkali metal cation increases. Consequently, the lattice energy steadily decreases in the order:

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

   Based strictly on ideal lattice energy, one would initially expect $$\text{LiCl}$$ to have the highest melting point.


2. Fajans' Rules (Covalent Character): Real ionic compounds possess varying degrees of covalent character. Fajans' Rules state that a small cation size has a exceptionally high charge density and polarizing power, allowing it to distort the electron cloud of the adjacent anion.
   • Because $$\text{Li}^+$$ is extremely small, it introduces significant covalent character into the $$\text{LiCl}$$ bond.
   • Compounds with high covalent character have lower melting points compared to purely ionic lattices. As a result, the melting point of $$\text{LiCl}$$ drops abnormally low ($$\sim 605^\circ\text{C}$$).

Once we pass lithium, the remaining alkali metal chlorides ($$\text{NaCl}$$, $$\text{KCl}$$, $$\text{RbCl}$$) behave like highly ideal ionic compounds with negligible polarization effects. For these three, the melting point trend follows the standard decrease in lattice energy as the cation grows larger:

$$\text{NaCl } (801^\circ\text{C}) > \text{KCl } (770^\circ\text{C}) > \text{RbCl } (718^\circ\text{C})$$

Combining all observations, the overall trend for the melting points of alkali metal chlorides is:

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

While $$\text{LiCl}$$ suffers a melting point depression due to its covalent character, $$\text{NaCl}$$ achieves the optimal balance of a powerful ionic lattice energy with negligible polarization, giving it the highest melting point in the series.

Correct Option: B — NaCl