Which of the following statement are correct for the ionic solids in which positive and negative ions are held by strong electrostatic attractive forces ?

Options

A. The radius ratio `r_(+)//r_(-)` increases as coordination number increases
B. As the difference in size of ions increases, coordination number increases
C. When coordination number is eight `r_(+)//r_(-)` ratio lies between `0.225 " to" 0.414`
D. In ZnS type structure, anions have ccp arrangement and cations occupy alternate tetrahedral voids. The coordination number of `Zn^(2+)` and `S^(2-)` respectively are 4 and 4

Explanation

To determine which statements are correct for ionic solids held by strong electrostatic attractive forces, we will analyze each statement based on the principles of ionic bonding and crystal structures.

### Step-by-Step Solution:

1. **Understanding Radius Ratio and Coordination Number:**
- The radius ratio \( r_{c^+}/r_{a^-} \) (where \( r_{c^+} \) is the radius of the cation and \( r_{a^-} \) is the radius of the anion) is crucial in determining the coordination number of ions in an ionic solid.
- As the radius ratio increases, the coordination number also increases. For example:
- A radius ratio of 0.155 corresponds to a coordination number of 3 (triangular void).
- A radius ratio of 0.732 corresponds to a coordination number of 8 (cubic void).

**Conclusion:** The first statement is **true**.

2. **Effect of Size Difference on Coordination Number:**
- The second statement claims that as the difference in size of ions increases, the coordination number increases.
- However, when the size difference increases, the radius ratio decreases, leading to a decrease in coordination number.

**Conclusion:** The second statement is **false**.

3. **Coordination Number and Radius Ratio:**
- The third statement suggests that when the coordination number is 8, the radius ratio \( r_{c^+}/r_{a^-} \) lies between 0.225 to 0.414.
- In reality, a radius ratio in this range corresponds to a coordination number of 4 (tetrahedral void), not 8.

**Conclusion:** The third statement is **false**.

4. **Zinc Blende Structure:**
- The fourth statement discusses the ZnS (zinc blende) structure, where anions are arranged in a close-packed (CCP) manner and cations occupy alternate tetrahedral voids.
- In this structure, both Zn²⁺ and S²⁻ have a coordination number of 4, which is correct.

**Conclusion:** The fourth statement is **true**.

### Final Answer:
The correct statements regarding ionic solids are:
- **Statement 1**: True
- **Statement 2**: False
- **Statement 3**: False
- **Statement 4**: True

Thus, the correct options are **A and D**.

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