A crystal of NaCl, which has sodium ions and chloride ions missing from the lattice point, is said to exhibit:

Options

A. Surface defect
B. Lattice defect
C. Frenkel defect
D. Schottky defect

Answer

To solve the question regarding the crystal of NaCl with missing sodium and chloride ions from the lattice points, we will analyze the types of defects that can occur in a crystal lattice.

### Step-by-Step Solution:

1. **Understanding the Crystal Structure of NaCl**:
- Sodium chloride (NaCl) forms a face-centered cubic (FCC) lattice structure where each sodium ion (Na⁺) is surrounded by six chloride ions (Cl⁻) and vice versa.

2. **Identifying the Types of Defects**:
- There are various types of lattice defects in solids, including:
- **Vacancy Defect (Wiccancy Defect)**: This occurs when some lattice sites are unoccupied.
- **Frankel Defect**: In this defect, an ion (cation or anion) moves from its normal lattice position to an interstitial site, creating a vacancy at the original site but maintaining the total number of ions.
- **Schottky Defect**: This defect occurs when equal numbers of cations and anions are missing from the lattice, maintaining charge neutrality.

3. **Analyzing the Given Situation**:
- The question states that both sodium ions and chloride ions are missing from the lattice points in the NaCl crystal.
- Since both types of ions are missing, we need to determine which defect this situation corresponds to.

4. **Determining the Correct Defect**:
- The vacancy defect does not apply here because it refers to unoccupied sites without specifying the type of ion missing.
- The Frankel defect does not apply because it involves the movement of ions to interstitial sites, which does not result in missing ions from the lattice.
- The Schottky defect is characterized by the absence of equal numbers of cations and anions from the lattice. Since both Na⁺ and Cl⁻ ions are missing, this situation perfectly describes the Schottky defect.

5. **Conclusion**:
- Therefore, the crystal of NaCl with sodium ions and chloride ions missing from the lattice points exhibits a **Schottky defect**.

### Final Answer:
- The correct answer is **Schottky defect** (Option 4).