Least stable peroxide among the following :

Options

A. `MgO_(2)`
B. `CaO_(2)`
C. `SrO_(2)`
D. `BaO_(2)`

Answer

To determine the least stable peroxide among the given options (MgO2, CaO2, SrO2, BaO2), we need to analyze the stability of these peroxides based on the size of the metal cations and their interaction with the peroxide anion (O2^2-).

### Step-by-Step Solution:

1. **Identify the Peroxides**: The given peroxides are MgO2, CaO2, SrO2, and BaO2. Each of these contains the peroxide anion (O2^2-).

2. **Understand the Size of Ions**: The stability of peroxides is influenced by the size of the metal cation. The cations involved are Mg^2+, Ca^2+, Sr^2+, and Ba^2+. As we move down the group in the periodic table, the size of the cations increases.

3. **Analyze the Interaction**: The larger the cation, the better it can stabilize the larger peroxide anion (O2^2-). This is because a larger cation can provide a better overlap with the anion, leading to stronger ionic interactions.

4. **Order the Cations by Size**: The order of the size of the cations from smallest to largest is:
- Mg^2+ < Ca^2+ < Sr^2+ < Ba^2+

5. **Determine Stability Based on Size**:
- BaO2 (with Ba^2+) will be the most stable because Ba^2+ is the largest cation.
- SrO2 (with Sr^2+) will be next in stability.
- CaO2 (with Ca^2+) will be less stable than SrO2.
- MgO2 (with Mg^2+) will be the least stable because Mg^2+ is the smallest cation.

6. **Conclusion**: Based on the size of the cations and their ability to stabilize the peroxide anion, the least stable peroxide among the given options is **MgO2**.

### Final Answer:
The least stable peroxide is **MgO2**.