Classification of Solids, Unit Cells, Packing Efficiency, Voids & Defects - Chemistry | Agrigyan

Defect characteristics of Potassium Chloride ($\text{KCl}$):

• Schottky Defect (A): $\text{KCl}$ is an ionic solid with high coordination number and cations ($\text{K}^+$) and anions ($\text{Cl}^-$) of comparable size, making it show Schottky defect (equal numbers of cation and anion vacancies).
• Metal Excess Defect (C): When $\text{KCl}$ crystals are heated in potassium vapor, excess potassium atoms deposit on the crystal surface and $\text{Cl}^-$ diffuses out, leaving an electron trapped in the anion vacancy ($F$-centre), which imparts a violet/lilac color. Thus, $\text{KCl}$ exhibits metal excess defect.
• $\text{KCl}$ does not show Frenkel defect (due to small size difference between ions) or metal deficiency defect (typical of transition metal oxides like $\text{FeO}$).

Characteristics of ionic solids:

• The constituent particles are cations and anions bound by strong electrostatic Coulombic forces (statement A is correct).
• They are hard and brittle, not soft (statement B is false).
• In the solid state, ions are fixed in their lattice positions and cannot move, so they act as electrical insulators (statement C is correct).
• In the molten state or when dissolved in water, ions become free to migrate, conducting electricity (statement D is correct).
• Due to strong Coulombic interactions, they possess high melting and boiling points (statement E is false).

Calculation steps:

• Let the number of atoms of element B forming the hcp lattice be $N$.
• The number of tetrahedral voids generated in a close-packed lattice is $2N$.
• Atoms of element A occupy $\frac{2}{3}$ of tetrahedral voids:
  $$\text{Number of A atoms} = \frac{2}{3} \times 2N = \frac{4}{3}N$$
• Ratio of atoms $\text{A} : \text{B}$:
  $$\text{A} : \text{B} = \frac{4}{3}N : N = 4 : 3$$