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 solve the question, we need to analyze the assertion and reason provided:

**Assertion (A)**: In NaCl, the coordination number of `Cl^(-)` ion is 6, but in CsCl, the coordination number of `Cl^(-)` ion is 8.

**Reason (R)**: Ionic radii change with the type of lattice.

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number of an ion in a crystal lattice refers to the number of nearest neighboring ions surrounding it. In NaCl, the `Cl^(-)` ion is surrounded by 6 `Na^+` ions, leading to a coordination number of 6. In CsCl, the `Cl^(-)` ion is surrounded by 8 `Cs^+` ions, leading to a coordination number of 8.

2. **Analyzing NaCl Structure**:
- NaCl has a face-centered cubic (FCC) structure where `Na^+` ions occupy the octahedral voids. The chloride ions are arranged in an FCC lattice. Each `Cl^(-)` ion touches 6 `Na^+` ions, confirming the coordination number of 6.

3. **Analyzing CsCl Structure**:
- CsCl has a simple cubic structure where `Cs^+` ions occupy the cubic voids. The `Cl^(-)` ions are arranged in a simple cubic lattice. Each `Cl^(-)` ion touches 8 `Cs^+` ions, confirming the coordination number of 8.

4. **Comparing Ionic Radii**:
- The assertion is correct as it accurately describes the coordination numbers in NaCl and CsCl. However, the reason states that ionic radii change with the type of lattice, which is misleading. The ionic radii themselves do not change; rather, the effective radius ratio (the ratio of cation to anion radii) influences the structure and coordination number.

5. **Conclusion**:
- The assertion is true, but the reason is false. Therefore, the correct option is that the assertion is true and the reason is false.

### Final Answer:
- The correct option is **Option 3**: Assertion is true, but Reason is false.

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