BaO has a rock-salt type structure. When subjected to high pressure, the ratio of the coordination number of `Ba^(+2)` ion to `O^(-2)` changes to

Options

A. `4 : 8`
B. `8 : 4`
C. `8 : 8`
D. `4 : 4`

Answer

To solve the problem regarding the change in the coordination number ratio of Ba²⁺ ions to O²⁻ ions in BaO when subjected to high pressure, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Structure of BaO**:
- Barium oxide (BaO) has a rock-salt type structure. In this structure, each Ba²⁺ ion is surrounded by O²⁻ ions and vice versa.

2. **Identifying the Coordination Numbers**:
- In the normal state (without high pressure), the coordination number of Ba²⁺ is 6, and the coordination number of O²⁻ is also 6. This gives us a ratio of:
\[ \text{Coordination number ratio} = \frac{6}{6} = 1 \]

3. **Effect of High Pressure**:
- When BaO is subjected to high pressure, the ions are forced closer together. This can lead to an increase in the coordination numbers.
- Under high pressure, the coordination number of Ba²⁺ can increase to 8, and the coordination number of O²⁻ can also increase to 8.

4. **Calculating the New Ratio**:
- After the increase in coordination numbers, the new ratio of the coordination number of Ba²⁺ to O²⁻ becomes:
\[ \text{New ratio} = \frac{8}{8} = 1 \]

5. **Conclusion**:
- Therefore, the ratio of the coordination number of Ba²⁺ to O²⁻ in BaO when subjected to high pressure changes to 1:1.

### Final Answer:
The ratio of the coordination number of Ba²⁺ to O²⁻ ions in BaO under high pressure is **1:1**.