Although crystalline solids have short range as well as long range order in the arrangement of their constituent particles, yet crystals are not perfect. Usually, a solid consists of an aggregate of large number of small crystals. These small crystals have defects in them. This happens when crystallization process occurs at fast or moderate rate. Single crystals are formed when the process of crystallization occurs at extremely slow rate. Even these crystals are not free of defects. The defects are basically irregularities in the arrangement of constituent particles. Broadly speaking, the defects are of two types, namely, point defects and line defects. Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance, whereas the line defects are the irregularities or deviations from ideal arrangement in entire rows of lattice points. These irregularities are called crystal defects.

If molten NaCl containing a little amount of SrCl₂ is crystallized, then crystal lattice will show which of the following point defects?

Impurity Defect

The correct answer is option 3. Impurity Defect.

When molten \(NaCl\) containing a small amount of \(SrCl_2\) is crystallized, the crystal lattice will show the impurity defect.

Let us analyze why the presence of \(SrCl_2\) in the \(NaCl\) crystal lattice causes an impurity defect.

An impurity defect occurs when foreign atoms (impurities) are present in the crystal lattice, either substituting for the host atoms or occupying interstitial sites.

In the case of \(NaCl\) with \(SrCl_2\):

\(SrCl_2\) dissociates into \( \text{Sr}^{2+} \) and \( 2\text{Cl}^{-} \).

The \( \text{Sr}^{2+} \) ion can replace \( \text{Na}^{+} \) ions in the NaCl crystal lattice.

Since \( \text{Sr}^{2+} \) has a +2 charge, replacing one \( \text{Na}^{+} \) ion (which has a +1 charge) would create a charge imbalance. Therefore, each \( \text{Sr}^{2+} \) ion would replace two \( \text{Na}^{+} \) ions to maintain electrical neutrality.

This substitution introduces impurity defects into the NaCl crystal lattice.

Other Point Defects:

1. Schottky Defect:

This defect involves missing pairs of cations and anions, maintaining overall electrical neutrality. It usually occurs in ionic crystals where cations and anions are of similar size.

The addition of \(SrCl_2\) does not directly cause Schottky defects.

2. Frenkel Defect:

This defect occurs when an ion (typically a cation) leaves its regular lattice site and moves to an interstitial site, creating a vacancy and an interstitial defect.

This type of defect typically occurs in crystals where there is a significant size difference between cations and anions.

\(SrCl_2\) addition does not induce Frenkel defects in \(NaCl\).

4. Metal Excess Defect:

This defect can arise from anionic vacancies or extra cations in interstitial sites, often balanced by electrons to maintain neutrality.

\(SrCl_2\) does not cause metal excess defects in \(NaCl\).

The presence of \(SrCl_2\) in the \(NaCl\) lattice introduces impurity ions (\(\text{Sr}^{2+}\)) into the lattice, replacing \(\text{Na}^{+}\) ions and creating impurity defects.