CaO and NaCl have the same crystal structure and approximately the same

ionic radii. If U is the lattice energy of NaCl, the approximate lattice energy of CaO is

a) U/2 b) U c) 4U d) 2U​

  Correct option  4U 

Explanation: 

q1 and q2 are charges on ions and r is the distance between them. Since interionic distances in CaO and NaCl are similar, (larger cation has smaller anion and vice versa) therefore, r is almost the same. Therefore, lattice energy depends only on charge. Since the magnitude of charge on Na+ and Cl- ions is unity and that on Ca2+ and Or ions is 2 each. therefore, the lattice energy of CaO is four times the lattice energy of NaCl, i.e., 4U.