Which of the following are true ?

Options

A. In NaCl crystals , `Na^+` ions are present in all the octahedral voids
B. In ZnS (zinc blende) , `Zn^(2+)` ions are present in alternate tetrahedral voids .
C. In `CaF_2, F^- ` ions occupy all the tetrahedral voids
D. In `Na_2O, O^(2-)` ions occupy half the octahedral voids.

Explanation

To determine which statements are true regarding the presence of ions in octahedral voids in various crystal structures, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding the Crystal Structure of NaCl:**
- In NaCl (sodium chloride), the structure consists of Na⁺ and Cl⁻ ions.
- The Na⁺ ions occupy the octahedral voids in the face-centered cubic (FCC) arrangement of Cl⁻ ions.
- **Conclusion:** The statement that Na⁺ ions are present in all octahedral voids is **true**.

2. **Analyzing the Presence of Zinc in Octahedral Voids:**
- Zinc can exist in different forms, such as zinc oxide (ZnO) and zinc sulfide (ZnS).
- In the case of the zinc blende structure, Zn²⁺ ions occupy half of the tetrahedral voids, while in the wurtzite structure, they occupy octahedral voids.
- **Conclusion:** The statement regarding zinc being present in all octahedral voids is **false**.

3. **Investigating NaCaF₂ Structure:**
- In NaCaF₂, the structure is based on a cubic arrangement where Ca²⁺ ions occupy the octahedral voids, and F⁻ ions occupy the tetrahedral voids.
- **Conclusion:** The statement that NaCaF₂ occupies all octahedral voids is **true**.

4. **Evaluating Na₂O Structure:**
- In Na₂O (sodium oxide), O²⁻ ions form a close-packed structure, and Na⁺ ions occupy the octahedral voids.
- However, not all octahedral voids are occupied by Na⁺ ions; some are left unoccupied.
- **Conclusion:** The statement that Na₂O occupies all octahedral voids is **false**.

### Summary of True Statements:
- The true statements are:
1. Na⁺ ions are present in all octahedral voids in NaCl.
2. NaCaF₂ occupies all octahedral voids.

### Final Answer:
The true statements are:
- NaCl has Na⁺ ions in all octahedral voids.
- NaCaF₂ occupies all octahedral voids.