Question

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

Answer 1

Lattice energy =−q1q2r2

Where q1 and q2 are charges on ions and r is the distance between them. Since inter-ionic distances in CaO and NaCl are similar, (larger cation has smaller anion and vice-versa) therefore ris 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 O2− ions is 2 each therefore, the lattice energy of CaO is four times the lattice energy of NaCl ie. 4U

Answer 2

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. ... therefore, the lattice energy of CaO is four times the lattice energy of NaCl, i.e., 4U