Assertion (A): `Na_(2)CO_(3)` and `Li_(2)CO_(3)` are thermally stable.
Reason (R ): Both the carbonates are salts of large cations and large anions.

Options

A. If both (A) and (R ) are correct and (R ) is the correct explanation of (A).
B. If both (A) and (R ) are correct, but (R ) is not the correct explanation of (A).
C. If (A) is correct, but (R ) is incorrect.
D. If (A) is incorrect, and (R ) is incorrect.

Explanation

To solve the assertion-reason question regarding the thermal stability of sodium carbonate (Na₂CO₃) and lithium carbonate (Li₂CO₃), we will analyze both the assertion and the reason step by step.

### Step 1: Analyze the Assertion
The assertion states that Na₂CO₃ and Li₂CO₃ are thermally stable.

- **Sodium Carbonate (Na₂CO₃)**: It is known to be thermally stable. When heated, it does not decompose and remains intact.
- **Lithium Carbonate (Li₂CO₃)**: In contrast, lithium carbonate is not thermally stable. Upon heating, it decomposes into lithium oxide (Li₂O) and carbon dioxide (CO₂).

**Conclusion for Assertion**: The assertion is incorrect because while Na₂CO₃ is thermally stable, Li₂CO₃ is not.

### Step 2: Analyze the Reason
The reason states that both carbonates are salts of large cations and large anions.

- **Cation Size**: Sodium ion (Na⁺) is larger than lithium ion (Li⁺). Therefore, Na⁺ can stabilize the large carbonate ion (CO₃²⁻) effectively.
- **Lithium Ion**: The lithium ion is small and cannot stabilize the large carbonate ion as effectively as sodium ion can. This leads to the thermal instability of Li₂CO₃.

**Conclusion for Reason**: The reason is also incorrect because it incorrectly categorizes lithium ion as a large cation. While the carbonate ion is large, the lithium ion is not.

### Final Conclusion
Both the assertion and the reason are incorrect.

### Answer: B) Both assertion and reason are incorrect.

---