A : NaCl shows Schottky defect at room temperature.
R : NaCl shows F centre at high temperature.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To analyze the given statements regarding NaCl and its defects, we will break down the assertion and reason step by step.

### Step 1: Understand the Assertion
The assertion states that "NaCl shows Schottky defect at room temperature."

- **Definition of Schottky Defect**: A Schottky defect is a type of stoichiometric defect that occurs in ionic crystals when an equal number of cations and anions are missing from the lattice. This defect maintains the overall stoichiometry of the crystal, meaning the ratio of cations to anions remains the same.

- **Application to NaCl**: In NaCl, both Na⁺ and Cl⁻ ions are of similar size, which allows for the formation of Schottky defects. At room temperature, NaCl can exhibit these defects as some ions may be thermally excited enough to leave their lattice positions.

### Step 2: Understand the Reason
The reason states that "NaCl shows F-centre at high temperature."

- **Definition of F-Centre**: An F-centre is a type of color center that occurs when an anion is missing from the lattice, leaving behind a vacancy that can trap an electron. This results in the crystal exhibiting color due to the presence of unpaired electrons that can absorb visible light.

- **Application to NaCl**: At high temperatures, NaCl can undergo thermal agitation that allows for the diffusion of anions (Cl⁻) out of the lattice. When these anions leave, they create vacancies which can be occupied by electrons, forming F-centres. This process leads to the coloration of the crystal.

### Step 3: Conclusion
Both the assertion and reason are correct:
- NaCl does show Schottky defects at room temperature due to the equal loss of cations and anions.
- NaCl can form F-centres at high temperatures due to the loss of anions and the trapping of electrons in the resulting vacancies.

### Final Statement
Thus, the correct answer is that both the assertion (A) and the reason (R) are true, but the reason does not adequately explain the assertion.

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