JEE Main 2021 — Chemical Bonding And Molecular Structure Question with Solution

Correct option is i.e. LiF > LiCl; MgO > NaCl. Melting point is directly proportional to lattice energy. Lattice energy is the energy required to separate a mole of an ionic solid into gaseous ions. It depends upon charge of ions and size of ions.

$$M.P.\propto L.E.\propto \frac{Charge}{Size}$$

\matrix{ {Li \to + 1} & {Li \to + 1} \cr {F = - 1} & {Cl \to - 1} \cr }

Both LiF and LiCl having same charge, so melting point will depend on size.

Larger the size of anion, lesser the lattice energy and hence, melting point order is LiF > LiCl.

Similarly, \matrix{ {MgO} & {NaCl} \cr {Mg \to 2 + } & {Na \to 1 + } \cr {O \to 2 - } & {Cl \to 1 - } \cr }

MgO having + 2 charge which is greater than NaCl (+ 1) charge. So, greater the charge on the ions greater will be lattice energy and hence, melting point order is MgO > NaCl.