Assertion (A) Beryllium carbonate is kept in the atomsphere of carbon dioxide.
Reason (R) Beryllium carbonate is unstable and decomposes to given beryllium oxide and carbon dioxide.

Options

A. Both A and R are correct and R is the correct explanation of A
B. Both A and R are correct but R is not the correct explanation of A
C. Both A and R are not correct
D. A is not correct but R is correct

Explanation

### Step-by-Step Solution

1. **Understanding the Assertion and Reason**:
- The assertion states that "Beryllium carbonate is kept in the atmosphere of carbon dioxide."
- The reason provided is that "Beryllium carbonate is unstable and decomposes to give beryllium oxide and carbon dioxide."

2. **Chemical Composition**:
- Beryllium carbonate is represented as \( \text{BeCO}_3 \).

3. **Decomposition of Beryllium Carbonate**:
- Beryllium carbonate decomposes into beryllium oxide and carbon dioxide:
\[ \text{BeCO}_3 \rightarrow \text{BeO} + \text{CO}_2 \]
- This reaction indicates that beryllium carbonate is indeed unstable.

4. **Reason for Instability**:
- The instability of beryllium carbonate can be attributed to the small size of the beryllium ion (\( \text{Be}^{2+} \)) compared to the carbonate ion (\( \text{CO}_3^{2-} \)).
- The small size of \( \text{Be}^{2+} \) leads to a strong polarization of the carbonate ion, making the compound unstable.

5. **Effect of Carbon Dioxide Atmosphere**:
- When beryllium carbonate is kept in an atmosphere of carbon dioxide, the increased concentration of \( \text{CO}_2 \) shifts the equilibrium of the decomposition reaction to the left:
\[ \text{BeO} + \text{CO}_2 \rightleftharpoons \text{BeCO}_3 \]
- This prevents the decomposition of beryllium carbonate, allowing it to remain stable in the presence of carbon dioxide.

6. **Conclusion**:
- Both the assertion and the reason are correct. The reason correctly explains why beryllium carbonate is kept in an atmosphere of carbon dioxide.

### Final Answer:
- Both assertion and reason are correct, and the reason is the correct explanation of the assertion.

---