Which of the following is incorrect statement ?

Options

A. Density decreases in case of crystals with Schottky 's defect.
B. `NaCl(s)` is insulator , silicon is semiconductor , silver is conductor , quartz is piezo electric crystal .
C. Frenkel defect is favoured in those ionic compounds in which sizes of cation and anions are almost equal.
D. `FeO_(0.98)` has non stoichiometric metal deficiency defect.

Answer

To solve the question of identifying the incorrect statement among the given options, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Short Key Defect**:
- In a Schottky defect, equal numbers of cations and anions are missing from the crystal lattice.
- When cations and anions are missing, the overall mass of the crystal decreases, leading to a decrease in density.
- **Conclusion**: The statement about density decreasing in the case of a Schottky defect is **correct**.

2. **Analyzing NaCl as an Insulator**:
- In the solid state, NaCl does not conduct electricity because it does not have free ions; thus, it behaves as an insulator.
- When dissolved in water, NaCl dissociates into Na⁺ and Cl⁻ ions, allowing it to conduct electricity.
- **Conclusion**: The statement that NaCl is an insulator in solid state is **correct**.

3. **Examining Frenkel Defect**:
- A Frenkel defect occurs when a cation leaves its normal lattice position and occupies an interstitial site, creating a vacancy.
- This defect typically occurs in ionic compounds where there is a significant size difference between cations and anions.
- The statement claims that Frenkel defects are favored in ionic compounds with a large size difference between cations and anions.
- **Conclusion**: This statement is **incorrect** because Frenkel defects usually occur in compounds where the sizes of cations and anions are similar.

4. **Identifying Metal Deficiency in Fe₀.₉₈O**:
- The formula Fe₀.₉₈O indicates a metal deficiency defect, where the metal (Fe) is less than the stoichiometric ratio.
- In this case, some Fe²⁺ ions are replaced by Fe³⁺ ions, leading to a deficiency of metal.
- **Conclusion**: The statement regarding the metal deficiency defect in Fe₀.₉₈O is **correct**.

### Final Answer:
The incorrect statement is option **3**.

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