Which of the following carbonate is thermally most unstable?

Options

A. `BeCO_(3)`
B. `CaCO_(3)`
C. `K_(2)CO_(3)`
D. `Rb_(2)CO_(3)`

Answer

To determine which carbonate is thermally most unstable among the given options (BeCO3, CaCO3, K2CO3, and Rb2CO3), we need to analyze the thermal stability of these carbonates based on their metal cations.

### Step-by-Step Solution:

1. **Identify the Group of Metals:**
- Beryllium (Be) and Calcium (Ca) belong to the alkaline earth metals (Group 2).
- Potassium (K) and Rubidium (Rb) belong to the alkali metals (Group 1).

2. **Understand Thermal Stability:**
- The thermal stability of carbonates generally increases as we move down the group in the periodic table. This is because larger cations can stabilize the carbonate ion better due to their lower charge density.

3. **Compare the Carbonates:**
- **BeCO3 (Beryllium Carbonate):** Beryllium has a high charge density and forms a less stable carbonate.
- **CaCO3 (Calcium Carbonate):** Calcium is larger than beryllium, so CaCO3 is more stable than BeCO3.
- **K2CO3 (Potassium Carbonate):** Potassium is an alkali metal, and its carbonate is more stable than both BeCO3 and CaCO3 due to the larger size and lower charge density.
- **Rb2CO3 (Rubidium Carbonate):** Rubidium is larger than potassium, making Rb2CO3 even more stable than K2CO3.

4. **Conclusion:**
- Among the carbonates listed, BeCO3 is the least stable due to the high charge density of beryllium, which makes its carbonate thermally unstable compared to the others.

### Final Answer:
**BeCO3 (Beryllium Carbonate) is thermally the most unstable among the given options.**