Which is the incorrect statement ?

Options

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

To determine which statement is incorrect regarding the solid state and defects in ionic compounds, we will analyze the statements provided in the question step by step.

### Step 1: Analyze the first statement regarding FeO0.98
- **Statement**: FeO0.98 has a non-stoichiometric metal deficiency defect.
- **Explanation**: In FeO, some Fe²⁺ ions are missing, and the charge is balanced by the presence of Fe³⁺ ions. This implies that Fe ions are reduced, while oxygen remains unchanged. The notation FeO0.98 suggests that there is a deficiency in the iron content, which is incorrect because FeO should ideally have a stoichiometric ratio of Fe:O = 1:1. Therefore, the statement is incorrect.

### Step 2: Analyze the second statement regarding Schottky Defect
- **Statement**: Density decreases in case of crystals with Schottky Defect.
- **Explanation**: Schottky defects occur when equal numbers of cations and anions are missing from the lattice, which leads to a decrease in density. This statement is correct.

### Step 3: Analyze the third statement regarding electrical properties of solids
- **Statement**: NaCl solid is an insulator; Silicon is a semiconductor; Silver is a conductor; Quartz is a piezoelectric crystal.
- **Explanation**: This statement correctly describes the electrical properties of these materials. NaCl is indeed an insulator, silicon is a semiconductor, silver is a good conductor, and quartz exhibits piezoelectric properties. Therefore, this statement is correct.

### Step 4: Analyze the fourth statement regarding Frankel defect
- **Statement**: Frankel defect is favored in ionic compounds where the size of cation and anion are almost equal.
- **Explanation**: Frankel defects occur when a cation leaves its position and occupies an interstitial site, which typically happens when there is a significant size difference between cations and anions. Therefore, this statement is incorrect because it suggests that Frankel defects occur in compounds with similar ion sizes.

### Conclusion
- The incorrect statements are:
1. The first statement about FeO0.98 being non-stoichiometric is incorrect.
2. The fourth statement about Frankel defect being favored in ionic compounds with similar ion sizes is also incorrect.

Thus, the answer to the question is that both the first and fourth statements are incorrect.