Based on lattice enthalpy and other considerations which one the following alkali metals chlorides is expected to have the higher melting point?

The correct option is NaCl.

To understand why sodium chloride (NaCl) has the highest melting point among the alkali metal chlorides (LiCl, NaCl, KCl, and RbCl), we must consider two competing factors: lattice enthalpy and covalent character (polarization).

1. Covalent Character and Fajans' Rules:
According to Fajans' rules, the covalent character in an ionic compound increases with a decrease in the size of the cation. A smaller cation has a higher charge density and thus a greater polarizing power, allowing it to distort the electron cloud of the larger anion (Cl^-).
Lithium (Li⁺) is the smallest cation among the alkali metals. Consequently, Li⁺ has a very high polarizing power, which introduces significant covalent character into lithium chloride (LiCl). Compounds with higher covalent character generally have lower melting points compared to purely ionic compounds. Thus, despite its small ionic size, LiCl has a lower melting point (approximately 605 °C) than expected.

2. Lattice Enthalpy of NaCl, KCl, and RbCl:
For the remaining chlorides—NaCl, KCl, and RbCl—the covalent character is negligible, and their properties are dominated by their ionic nature. For these purely ionic compounds, the melting point is directly related to the lattice enthalpy. The lattice enthalpy (U) of an ionic crystal is inversely proportional to the sum of the radii of the cation (r₊) and anion (r_-):

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

Since the size of the alkali metal cations increases down the group:
Na⁺ < K⁺ < Rb⁺
The interionic distance is smallest for NaCl. As a result, the lattice enthalpy is highest for NaCl, making its crystal lattice the most stable and difficult to break. This gives NaCl the highest melting point among them.

Conclusion:
Combining both factors, the melting point trend for these alkali metal chlorides is:
NaCl (801 °C) > KCl (770 °C) > RbCl (718 °C) > LiCl (605 °C)
Therefore, NaCl has the highest melting point.