Question

Which is the incorrect statement?

• A. FeO_0.98 has non stoichiometric metal deficiency defect
• B. Density decreases in case of crystals with Schottky's defect
• C. NaCl(s) is insulator, silicon is semiconductor, silver is conductor, quartz is piezo electric crystal
• D. Frenkel defect is favoured in those ionic compounds in which sizes of cation and anions are almost equal

Answer 1

The Correct Option is A, D

To determine which statements are incorrect, let's evaluate each option by understanding the relevant defects and crystal properties: 

1. Statement 1: FeO_0.98 has a non-stoichiometric metal deficiency defect.
   • This statement is correct. In FeO_0.98, the ratio of Fe to O is not exactly 1:1, indicating a non-stoichiometric defect. Specifically, there is a deficiency of iron (Fe), meaning some iron ions are missing or replaced by other metal ions, leading to a metal deficiency defect.
2. Statement 2: Density decreases in case of crystals with Schottky's defect.
   • This statement is correct. Schottky defects occur when equal numbers of cations and anions are missing from the lattice, reducing the overall number of particles in the crystal without altering the stoichiometry. This loss of ions results in a decrease in the overall density of the crystal.
3. Statement 3: NaCl(s) is insulator, silicon is semiconductor, silver is conductor, quartz is piezoelectric crystal.
   • This statement is correct. NaCl is an electrical insulator as a solid; silicon is a well-known semiconductor; silver is a good electrical conductor; quartz exhibits piezoelectric properties, meaning it can generate an electric charge in response to applied mechanical stress.
4. Statement 4: Frenkel defect is favoured in those ionic compounds in which sizes of cation and anions are almost equal.
   • This statement is incorrect. Frenkel defects occur when a smaller ion (usually a cation) displaces from its lattice site to an interstitial position. This defect is favored in ionic compounds where there is a significant size difference between cations and anions, not when they are similar in size.

Conclusion: The incorrect statement is: "Frenkel defect is favoured in those ionic compounds in which sizes of cation and anions are almost equal."