Question

$CaO$ 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. 2U
• D. 4U

Answer 1

Answer: (D)

Lattice energy $=-\frac{q_1{q}_2}{r^2}$


where $q_1$ and $q_2$ 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 $N{a}^{+}$ and $C{l}^{-}$ ions is unity and that on $C{a}^{2+}$ and $O^{2-}$ ions is 2 each, therefore, the lattice energy of CaO is four times the lattice energy of NaCl, i.e.,4U.