Assertion (A) : Non-stoichiometric compounds are called Bertholide compounds. `NaCl` and `KCl` crystal, when heated in an atmosphere of `Na` and `K` varpours, respectively, they impart violet and yellow colours to `NaCl` and `KCl`, respectively.
Reason (R ) : Metal excess defect is due to the presence of extra cations atg the interstitial sites. The excess metal ions move to the interstitial sites and the electrons to the neighbouring sites. The colour results byt the excitation of these electrons by absorbing suitable energy from visible light. When the excited electroon comes back to the ground state, there is emission of radiation in the visible region and gives complimentary colour.

Options

A. If both `(A)` and `(R)` are correct, and `(R)` is the correct explanation of `(A)`
B. If both `(A)` and `(R)` are correct, but `(R)` is not the correct explanation of `(A)`
C. If `(A)` is correct, but `(R)` is incorrect.
D. If both `(A)` is incorrect, but `(R)` is correct.

Explanation

To solve the given assertion-reason question, we will analyze both the assertion (A) and the reason (R) step by step.

### Step 1: Analyze the Assertion (A)
The assertion states: "Non-stoichiometric compounds are called Bertholide compounds. NaCl and KCl crystals, when heated in an atmosphere of Na and K vapors, respectively, impart violet and yellow colors to NaCl and KCl, respectively."

- **Evaluation of Assertion**:
- The first part of the assertion is incorrect. Non-stoichiometric compounds are not called Bertholide compounds; they are referred to as "Berthollide" compounds.
- The second part of the assertion is also incorrect regarding the colors. NaCl actually imparts a yellow color, while KCl imparts a violet color when heated in their respective metal vapors.

### Conclusion for Assertion:
- The assertion is **incorrect**.

### Step 2: Analyze the Reason (R)
The reason states: "Metal excess defect is due to the presence of extra cations at the interstitial sites. The excess metal ions move to the interstitial sites and the electrons to the neighboring sites. The color results from the excitation of these electrons by absorbing suitable energy from visible light. When the excited electron comes back to the ground state, there is emission of radiation in the visible region and gives complementary color."

- **Evaluation of Reason**:
- The explanation provided in the reason is correct. Metal excess defects occur when extra cations occupy interstitial sites, and to maintain charge neutrality, electrons move to neighboring sites.
- The excitation of these electrons by visible light and the subsequent emission of radiation, which results in the observed color, is also accurately described.

### Conclusion for Reason:
- The reason is **correct**.

### Final Conclusion:
- Since the assertion is incorrect and the reason is correct, the answer to the assertion-reason question is:
- **Assertion is incorrect, Reason is correct.**

### Answer:
4. Reason is correct and Assertion is incorrect.

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