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)`

Answer

To determine which carbonate is unstable in air and needs to be kept in a CO₂ atmosphere to avoid decomposition, we can analyze the stability of the given carbonates: BeCO₃, MgCO₃, CaCO₃, and BaCO₃.

### Step-by-Step Solution:

1. **Understanding Carbonate Decomposition**:
- Metal carbonates generally decompose upon heating to form metal oxides and carbon dioxide (CO₂). The general reaction can be represented as:
\[ \text{MCO}_3 \rightarrow \text{MO} + \text{CO}_2 \]
- Here, M represents the metal.

2. **Identifying the Carbonates**:
- The carbonates provided in the options are:
- A) BeCO₃ (Beryllium Carbonate)
- B) MgCO₃ (Magnesium Carbonate)
- C) CaCO₃ (Calcium Carbonate)
- D) BaCO₃ (Barium Carbonate)

3. **Analyzing Stability**:
- **Beryllium Carbonate (BeCO₃)**:
- Beryllium carbonate is known to be unstable in air. It decomposes easily into beryllium oxide (BeO) and carbon dioxide (CO₂).
- The decomposition reaction is:
\[ \text{BeCO}_3 \rightarrow \text{BeO} + \text{CO}_2 \]
- The carbonate ion (CO₃²⁻) is relatively large compared to the small beryllium ion (Be²⁺), leading to instability in the compound.

- **Magnesium Carbonate (MgCO₃)**:
- Magnesium carbonate is more stable and does not decompose as readily as beryllium carbonate.

- **Calcium Carbonate (CaCO₃)**:
- Calcium carbonate is also stable and is commonly found in nature (e.g., limestone).

- **Barium Carbonate (BaCO₃)**:
- Barium carbonate is relatively stable as well and does not require special conditions to prevent decomposition.

4. **Conclusion**:
- Among the given options, **Beryllium Carbonate (BeCO₃)** is the only one that is unstable in air and is kept in a CO₂ atmosphere to avoid decomposition. Therefore, the answer is:
\[ \text{Answer: A) BeCO}_3 \]