Question

Which of the following statement(s) is/are true regarding defects in ionic solids?

• A. Heating $NaCl$ in sodium vapour atmosphere produces impurity defect.
• B. F-centres are created in Frenkel defect
• C. Adding $SrC{l}_2$ to molten $NaCl$ produces impurity defect
• D. Irregularity around a line is called line defects

Answer 1

Answer: (C), (D)

Irregularity around a line is called line defects

Defect is a disruption in the periodic order of a crystalline material. It occurs in crystals to lower their energy and increase thermodynamic stability.
Defects are of two types,
1. Point defect
2. Line defect

Irregularity or deviation from ideal arrangement around a point or a particle is called point defect.

Irregularity around a line or in entire rows of lattice points is called line defects.

Defects in ionic crystals, can be introduced by adding impurities
in which the ions are in different valence state than the constituent ions of the crystal. These defects are called impurity defects.

When small amounts of strontium chloride $\left(SrC{l}_2\right)$ are added to molten sodium chloride $\left(NaCl\right)$, some $S{r}^{2+}$ ions replace $N{a}^{+}$ ions and occupy the sites earlier occupied by the $N{a}^{+}$ ions. Each $S{r}^{2+}$ replaces two $N{a}^{+}$ ions by occupying a site and other site remaining vacant. Here, F-centres (anionic sites occupied by unpaired electrons) are not created.

Heating $NaCl$ in sodium vapour atmosphere gives metal excess defect.

Thus, statement (a) and (c) are true.