Which of the following order is correct for thermal stability?

Options

A. `MgCO_(3)lt CaCO_(3)ltBeCO_(3)`
B. `BeCO_(3)lt MgCO_(3)ltCaCO_(3)`
C. `CACO_(3)lt BeCO_(3)lt MgCO_(3)`
D. `CACO_(3)lt MgCO_(3)lt BeCO_(3)`

Answer

To determine the correct order of thermal stability for the given carbonates of group 2 metals (Beryllium, Magnesium, and Calcium), we need to analyze the thermal stability of each carbonate based on the size of the metal cations and their interaction with the carbonate anion.

### Step-by-Step Solution:

1. **Understanding Thermal Stability**:
- Thermal stability refers to the ability of a compound to resist decomposition when heated. A compound that decomposes at a higher temperature is considered more thermally stable.

2. **Identifying the Compounds**:
- The carbonates we are considering are Beryllium carbonate (BeCO₃), Magnesium carbonate (MgCO₃), and Calcium carbonate (CaCO₃).

3. **Analyzing the Cation Sizes**:
- In Group 2 (alkaline earth metals), as we move down the group from Beryllium to Calcium, the size of the cations increases:
- Beryllium (Be²⁺) < Magnesium (Mg²⁺) < Calcium (Ca²⁺)

4. **Effect of Cation Size on Stability**:
- The stability of the carbonate compounds depends on the effective overlap between the cation and the carbonate anion (CO₃²⁻). A better size match between the cation and the anion leads to stronger bonding and thus higher thermal stability.
- Beryllium, being the smallest cation, does not stabilize the carbonate well due to poor overlap with the larger carbonate anion. Therefore, Beryllium carbonate is less stable.
- Magnesium carbonate, while larger than Beryllium, still does not have as good a size match as Calcium carbonate, which has the largest cation and a better overlap with the carbonate anion.

5. **Ranking the Thermal Stability**:
- Based on the above analysis:
- Calcium carbonate (CaCO₃) has the best size match and thus the highest thermal stability.
- Magnesium carbonate (MgCO₃) is more stable than Beryllium carbonate (BeCO₃) but less stable than Calcium carbonate.
- Therefore, the order of thermal stability is:
- **CaCO₃ > MgCO₃ > BeCO₃**

6. **Conclusion**:
- The correct order for thermal stability of the carbonates is:
- **Calcium carbonate > Magnesium carbonate > Beryllium carbonate**.