JEE Main · 2021 · 24 Feb 2021 · Shift 2 · Chemical Bonding and Molecular Structure

The melting point of an ionic solid follows its lattice enthalpy, which grows with the product of the ionic charges and falls as the interionic distance increases. The fluoride ion is much smaller than the chloride ion, so the cation-anion distance in LiF is shorter and its lattice enthalpy larger, placing the melting point of LiF above that of LiCl. In MgO both ions carry a double charge while NaCl is built from singly charged ions, and the fourfold larger charge product raises the lattice enthalpy of MgO far above that of NaCl. Both orders $\mathrm{LiF}>\mathrm{LiCl}$ and $\mathrm{MgO}>\mathrm{NaCl}$ are therefore the correct ones.