Question

CaO and NaCl have the same crystal structure and approximately the same ionic radii. If U is the Iattice energy of NaCl, the approximate lattice energy of CaO is

(a) U/2

(b) U

(c) 2U

(d) 4U

Answer 1

Correct option  (d) 4U

Explanation:

where 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.