A : The co - ordination number of `CaF_(2)` is 8 : 4.
R : `Ca^(2+)` ions occupy ccp lattice while `F^(-)` ions occupy 50% octahedral voids and 50% tetrachedral voids.

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 problem, we need to analyze the assertion and reason provided regarding the coordination number and the structure of calcium fluoride (CaF₂).

### Step-by-Step Solution:

1. **Understanding the Assertion:**
- The assertion states that the coordination number of CaF₂ is 8:4.
- The coordination number refers to the number of nearest neighbor ions surrounding a central ion in a crystal lattice.

2. **Analyzing the Structure of CaF₂:**
- In the crystal structure of CaF₂, calcium ions (Ca²⁺) occupy a cubic close-packed (CCP) lattice.
- In a CCP structure, each calcium ion is surrounded by 8 fluoride ions (F⁻), giving it a coordination number of 8.

3. **Determining the Position of Fluoride Ions:**
- The fluoride ions occupy the voids in the CCP lattice.
- Specifically, fluoride ions occupy the tetrahedral voids in the lattice.
- In a CCP structure, there are 8 tetrahedral voids per unit cell, and since each Ca²⁺ ion is coordinated to 4 F⁻ ions, the coordination number of F⁻ ions is 4.

4. **Conclusion about the Assertion:**
- Therefore, the assertion that the coordination number of CaF₂ is 8:4 is true.

5. **Understanding the Reason:**
- The reason states that Ca²⁺ ions occupy a CCP lattice while F⁻ ions occupy 50% octahedral voids and 50% tetrahedral voids.
- In fact, in the case of CaF₂, the fluoride ions occupy only the tetrahedral voids, not octahedral voids.
- Thus, the statement regarding F⁻ ions occupying octahedral voids is false.

6. **Final Evaluation:**
- Since the assertion is true and the reason is false, we conclude that the correct option is that the assertion is true, but the reason is false.

### Final Answer:
- The assertion is true, and the reason is false.