Which of the carbonates given below is unstable in air and is kept in `CO_(2)` atmosphere to avoid decomposition?

Options

A. `BeCO_(3)`
B. `MgCO_(3)`
C. `CaCO_(3)`
D. `BaCO_(3)`

Explanation

To solve the question regarding which carbonate is unstable in air and is kept in a CO₂ atmosphere to avoid decomposition, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Carbonates**: The question mentions several carbonates, but we need to focus on their stability in air. The carbonates typically considered in this context are those of alkaline earth metals, particularly Beryllium, Magnesium, Calcium, Strontium, and Barium.

2. **Understand Beryllium Carbonate**: Beryllium carbonate (BeCO₃) is known to be unstable. It decomposes easily when exposed to air, breaking down into beryllium oxide (BeO) and carbon dioxide (CO₂).

3. **Decomposition Reaction**: The decomposition reaction for beryllium carbonate can be written as:
\[ \text{BeCO}_3 \rightarrow \text{BeO} + \text{CO}_2 \]
This indicates that beryllium carbonate breaks down into more stable products (beryllium oxide and carbon dioxide).

4. **Role of Carbon Dioxide Atmosphere**: When beryllium carbonate is kept in an atmosphere rich in carbon dioxide, the reaction can shift back to the left due to Le Chatelier's principle. This means that the presence of CO₂ helps to prevent the decomposition of beryllium carbonate, allowing it to remain stable.

5. **Conclusion**: Therefore, the carbonate that is unstable in air and is kept in a CO₂ atmosphere to avoid decomposition is **Beryllium Carbonate (BeCO₃)**.

### Final Answer:
The correct answer is **Beryllium Carbonate (BeCO₃)**.