The thermal stability of alkaline earth metal carbonates `MgCO_(3),CaCO_(3),BaCO_(3)` and `SrCO_(3)` decreases as:

Options

A. `CaCO_(3) gt SrCO_(3) gt MgCO_(3) gt BaCO_(3)`
B. `BaCO_(3) gt SrCO_(3) gt MgCO_(3) gt CaCO_(3)`
C. `BaCO_(3) gt SrCO_(3) gt CaCO_(3) gt MgCO_(3)`
D. `MgCO_(3) gt CaCO_(3) gt SrCO_(3) gt BaCO_(3)`

Answer

To determine the thermal stability of alkaline earth metal carbonates \( \text{MgCO}_3, \text{CaCO}_3, \text{SrCO}_3, \) and \( \text{BaCO}_3 \), we need to analyze how their thermal stability changes as we move down the group in the periodic table.

### Step-by-Step Solution:

1. **Understanding Thermal Stability**:
- Thermal stability refers to the ability of a compound to withstand heat without decomposing. In this case, we are looking at the carbonates of alkaline earth metals.

2. **General Trend in Alkaline Earth Metal Carbonates**:
- The thermal stability of alkaline earth metal carbonates generally increases as we move down the group from magnesium to barium. This is due to the increasing size of the cation.

3. **Ionic Character and Polarizing Power**:
- The stability of the carbonates is influenced by the ionic character of the metal-carbonate bond. As the size of the cation increases (from Mg to Ba), the polarizing power of the cation decreases.
- Smaller cations (like Mg²⁺) have a higher charge density and can polarize the carbonate ion (CO₃²⁻) more effectively, leading to a greater covalent character in the bond.

4. **Covalent vs. Ionic Character**:
- A higher covalent character in the bond results in lower thermal stability. Thus, as we move down the group, the increasing size of the cation leads to a decrease in polarizing power, which results in a decrease in covalent character and an increase in ionic character.

5. **Conclusion**:
- Therefore, the thermal stability of the carbonates decreases in the order:
\[ \text{BaCO}_3 > \text{SrCO}_3 > \text{CaCO}_3 > \text{MgCO}_3 \]
- This means that \( \text{MgCO}_3 \) is the least thermally stable, while \( \text{BaCO}_3 \) is the most thermally stable.

### Final Answer:
The thermal stability of alkaline earth metal carbonates decreases in the order:
\[ \text{MgCO}_3 < \text{CaCO}_3 < \text{SrCO}_3 < \text{BaCO}_3 \]