A : In NaCl coordination number of `Cl^(-)` ion is 6 but in CsCl coordination number of `Cl^(-)` ion is 8.
R : Ionic radii changes with type of lattice.

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 assertion (A) and reason (R) provided in the question, we will break down the concepts of coordination number and ionic radii in the context of NaCl and CsCl.

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number of an ion in a crystal lattice refers to the number of nearest neighbor ions surrounding it. In NaCl, the coordination number of the Cl⁻ ion is 6, while in CsCl, it is 8.

2. **Analyzing NaCl**:
- In NaCl, the Na⁺ ions occupy octahedral voids in a face-centered cubic (FCC) lattice formed by Cl⁻ ions. Each Cl⁻ ion is surrounded by 6 Na⁺ ions, leading to a coordination number of 6.

3. **Analyzing CsCl**:
- In CsCl, the Cs⁺ ions occupy cubic voids in a simple cubic lattice formed by Cl⁻ ions. Each Cl⁻ ion is surrounded by 8 Cs⁺ ions, resulting in a coordination number of 8.

4. **Ionic Radii and Lattice Type**:
- The ionic radii of ions can vary based on the type of lattice they are in. The ratio of the ionic radii (R₊/R₋) helps determine the type of lattice structure formed.
- For NaCl, the ratio of the ionic radii (Na⁺ to Cl⁻) is between 0.414 and 0.7, indicating octahedral coordination.
- For CsCl, the ratio is approximately 0.732, suggesting a cubic coordination.

5. **Conclusion**:
- The assertion (A) is true: Cl⁻ has a coordination number of 6 in NaCl and 8 in CsCl.
- The reason (R) is also true: Ionic radii change with the type of lattice, which influences the coordination number.

### Final Statement:
Both the assertion (A) and reason (R) are true, and the reason correctly explains the assertion.

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