Q.What do you understand by point defects?

Frenkel defect needs a large cation–anion size mismatch so the smaller ion can slip into an interstitial void; in NaCl the two ions are close enough in size that this does not happen, so option (a) is correct.

Why Frenkel defect needs a size mismatch

A Frenkel defect arises when a smaller ion (usually the cation) leaves its regular lattice site and occupies an interstitial position, creating a vacancy at its original site and an interstitial ion elsewhere — the overall stoichiometry and electrical neutrality of the crystal are preserved. This defect is favoured only when there is a large difference in the sizes of the cation and anion, because a small cation can then be accommodated in the small interstitial voids of the anion lattice without much strain. Classic examples are $AgBr$, $AgCl$, $AgI$ and $ZnS$, where $Ag^+$/$Zn^{2+}$ are much smaller than $Br^-$/$Cl^-$/$S^{2-}$.

Why NaCl is different

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A point defect is a localised irregularity at a single lattice site in an otherwise perfectly ordered crystal, unlike line or planar defects which extend over many lattice points.

Point defects in crystals

An ideal crystalline solid has its constituent particles (atoms, ions, or molecules) arranged in a perfectly ordered, repeating three-dimensional pattern. In real crystals, however, deviations from this perfect order occur at isolated points in the lattice, called point defects. They arise mainly due to thermal vibrations (some particles gain enough energy to leave their regular sites) during crystallisation.

Common types of point defects include:

• Vacancy defect — a lattice site that is unoccupied (an atom/ion is missing from its position); this lowers the density of the solid.
• Interstitial defect — an extra particle occupies a normally empty interstitial (in-between) site, increasing the density.
• Impurity defect — a foreign atom/ion occupies a regular lattice site or an interstitial site (this is the basis of doping). …