Assertion : In `CaF_2, F^(-)` ions occupy all the tetrahedral sites.
Reason : The number of `Ca^(2+)` is double the number of `F^(-)` ions.

Explanation

To solve the question, we need to analyze the assertion and the reason given regarding the structure of calcium fluoride (CaF₂).

### Step-by-Step Solution:

1. **Understanding the Structure of CaF₂**:
- Calcium fluoride (CaF₂) has a face-centered cubic (FCC) structure. In this structure, the calcium ions (Ca²⁺) occupy the cubic lattice points, and the fluoride ions (F⁻) occupy the tetrahedral and octahedral voids.

2. **Determining the Number of Ions in the Unit Cell**:
- In a face-centered cubic unit cell, there are 4 formula units of CaF₂. This is because each unit cell contains 4 calcium ions and 8 fluoride ions.
- The formula unit of CaF₂ indicates that for every 1 calcium ion, there are 2 fluoride ions.

3. **Identifying the Tetrahedral and Octahedral Sites**:
- In the FCC structure, there are 8 tetrahedral voids and 4 octahedral voids per unit cell.
- Since each Ca²⁺ ion is surrounded by 4 F⁻ ions in a tetrahedral arrangement, the F⁻ ions occupy all the tetrahedral sites.

4. **Analyzing the Assertion and Reason**:
- **Assertion**: "In CaF₂, F⁻ ions occupy all the tetrahedral sites." This is true because there are 8 tetrahedral voids available and 8 F⁻ ions in the unit cell.
- **Reason**: "The number of Ca²⁺ is double the number of F⁻ ions." This is false because in CaF₂, the number of F⁻ ions is actually double the number of Ca²⁺ ions (8 F⁻ ions for 4 Ca²⁺ ions).

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

### Final Answer:
- Assertion: True
- Reason: False