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

Options

A. `BaCO_(3) gt CaCO_(3) gt MgCO_(3)`
B. `BaCO_(3) gt MgCO_(3) gt CaCO_(3)`
C. `CaCO_(3) gt MgCO_(3) gt BaCO_(3)`
D. `MgCO_(3) gt CaCO_(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 can analyze how the 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 resist decomposition when heated. In this case, we are looking at the thermal stability of alkaline earth metal carbonates.

2. **Group Trend in Alkaline Earth Metals**:
- The alkaline earth metals include Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), and Barium (Ba). As we move down the group from Be to Ba, the size of the metal cation increases.

3. **Effect of Cation Size on Stability**:
- Larger cations (like Ba) tend to form more stable carbonates compared to smaller cations (like Mg). This is because the larger cations can better stabilize the carbonate ion (\( \text{CO}_3^{2-} \)) through lattice energy.

4. **Order of Thermal Stability**:
- The thermal stability of the carbonates increases down the group. Therefore, we can arrange the carbonates in order of increasing thermal stability:
- \( \text{MgCO}_3 < \text{CaCO}_3 < \text{SrCO}_3 < \text{BaCO}_3 \)

5. **Conclusion**:
- Thus, the thermal stability of alkaline earth metal carbonates decreases in the order:
\[ \text{BaCO}_3 > \text{SrCO}_3 > \text{CaCO}_3 > \text{MgCO}_3 \]

### Final Answer:
The correct order of thermal stability is:
\[ \text{BaCO}_3 > \text{SrCO}_3 > \text{CaCO}_3 > \text{MgCO}_3 \]