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Q.
$CaO$ 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
AIIMSAIIMS 2010Chemical Bonding and Molecular Structure
Solution:
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 $Na^+$ and $Cl^-$ ions is unity and that on $Ca^{2+}$ and $O^{2-}$ ions is 2 each, therefore, the lattice energy of CaO is four times the lattice energy of NaCl, i.e.,4U.