Q1(a). Defect in silver chloride

Answer in brief

AgCl shows the Frenkel defect: some Ag⁺ ions leave their lattice sites and sit in interstitial sites.

Key points

• Size: Ag⁺ (115 pm) is much smaller than Cl⁻ (181 pm), so it fits an interstitial hole.
• Polarisation: Ag⁺ (18-electron shell) is soft and polarisable; covalent character stabilises it in the odd site.
• Dielectric: high dielectric constant screens the vacancy–interstitial charges, lowering ΔH_f.
• Number of defects n ≈ √(N·Nᵢ)·e^−ΔH_f/2kT rises steeply with T; density is unchanged (stoichiometric defect).