The most thermally unstable carbonate among the following is

Options

A. `CaCO_3`
B. `MgCO_3`
C. `BeCO_3`
D. `BaCO_3`

Answer

To determine the most thermally unstable carbonate among the given options (CaCO3, MgCO3, BeCO3, BaCO3), we will analyze the thermal stability of carbonates of Group 2 elements.

### Step-by-Step Solution:

1. **Identify the Carbonates**: The carbonates given are:
- CaCO3 (Calcium Carbonate)
- MgCO3 (Magnesium Carbonate)
- BeCO3 (Beryllium Carbonate)
- BaCO3 (Barium Carbonate)

2. **Understand Thermal Decomposition**: All carbonates decompose upon heating to form metal oxides (MO) and carbon dioxide (CO2). The general reaction can be represented as:
\[ MCO3 \rightarrow MO + CO2 \]

3. **Analyze Group Trends**: In Group 2 (alkaline earth metals), as we move down the group from Beryllium (Be) to Barium (Ba), the thermal stability of the carbonates increases. This is due to the following reasons:
- **Electropositive Character**: The electropositive character of the metal increases down the group.
- **Charge Density**: The size of the metal ion increases, leading to a decrease in charge density.
- **Polarizing Power**: A decrease in charge density results in lower polarizing power, which means the metal ion distorts the carbonate ion less.

4. **Conclusion on Stability**:
- Beryllium (Be) has the smallest size and highest charge density, making it capable of distorting the carbonate ion significantly. This leads to a weaker bond between Be and CO3.
- As a result, BeCO3 is the least thermally stable and will decompose at lower temperatures compared to the other carbonates.

5. **Final Answer**: Therefore, the most thermally unstable carbonate among the options is **BeCO3 (Beryllium Carbonate)**.