Chemical Bonding and Molecular Structure Part 1

Correct option is (a) 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∝L.E‌∝‌

‌ Charge ‌

‌ Size ‌

‌
Li→+1‌Li→+1
F=−1‌Cl→−1
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, MgONaCl
Mg→2+‌Na→1+
O→2−‌Cl→1−
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.