Schottky defect is observed in crystals when ……………. .

Options

A. some cations move from their lattice site to intersititial sites
B. equal number of cations and anions are missing form the lattice
C. some lattice sites are occupied by electrons
D. some impurity is present in the lattice

Explanation

**Step-by-Step Solution:**

1. **Understanding Defects in Crystals:**
- In solid-state chemistry, defects in crystal lattices can be classified into two main types: Schottky defects and Frenkel defects.

2. **Defining Schottky Defect:**
- A Schottky defect occurs when an equal number of cations and anions are missing from the crystal lattice. This results in vacancies that lead to a decrease in the density of the crystal.

3. **Visualizing the Lattice:**
- Imagine a crystal lattice where cations and anions are arranged in a specific pattern. If one cation and one anion are missing from their respective lattice sites, this creates a Schottky defect.

4. **Impact on Density:**
- The removal of equal numbers of cations and anions causes a reduction in the overall mass of the crystal without changing its volume significantly, leading to a decrease in density.

5. **Evaluating Options:**
- When presented with options regarding the conditions under which Schottky defects occur:
- Option 1: "Some cations move from the lattice site to the interstitial sites" - This describes a Frenkel defect, not a Schottky defect.
- Option 2: "Equal number of cations and anions are missing from the lattice" - This is the correct description of a Schottky defect.
- Option 3: "Some lattice sites are occupied by electrons" - This does not pertain to Schottky defects.
- Option 4: "Some impurities present in the lattice" - This refers to impurity defects.

6. **Conclusion:**
- The correct answer is that Schottky defects are observed when an equal number of cations and anions are missing from the lattice.

**Final Answer:**
Schottky defect is observed in crystals when equal number of cations and anions are missing from the lattice.

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