Metal carbonates decompose on heating to give metal oxide and carbon dioxide. Which of the metal carbonates is most stable thermally?

Options

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

Explanation

To determine which metal carbonate is the most thermally stable, we need to analyze the stability of the given metal carbonates when heated. The metal carbonates provided are:

A) Magnesium carbonate (MgCO₃)
B) Calcium carbonate (CaCO₃)
C) Strontium carbonate (SrCO₃)
D) Barium carbonate (BaCO₃)

### Step-by-Step Solution:

1. **Understanding Thermal Decomposition**:
Metal carbonates decompose upon heating to form metal oxides and carbon dioxide gas. The general reaction can be represented as:
\[ \text{MCO}_3 \rightarrow \text{MO} + \text{CO}_2 \]
where M represents the metal.

2. **Analyzing Stability**:
The stability of metal carbonates generally decreases as we move down the group in the periodic table. This is because larger cations tend to form more stable carbonates due to the larger size of the carbonate ion (CO₃²⁻).

3. **Comparing Sizes of Cations**:
The cation sizes for the alkaline earth metals increase as we move down the group:
- Beryllium (Be²⁺) < Magnesium (Mg²⁺) < Calcium (Ca²⁺) < Strontium (Sr²⁺) < Barium (Ba²⁺)
Thus, Ba²⁺ is the largest cation among the options given.

4. **Stability Trend**:
- Smaller cations (like Be²⁺ and Mg²⁺) form more stable carbonates due to better lattice energy.
- Larger cations (like Ba²⁺) form less stable carbonates because the lattice energy decreases as the size of the cation increases.

5. **Identifying the Most Stable Carbonate**:
- Among the given options, barium carbonate (BaCO₃) is the least stable thermally because it decomposes at lower temperatures compared to the others.
- Conversely, magnesium carbonate (MgCO₃) is more stable thermally than barium carbonate.

6. **Conclusion**:
Based on the analysis, the most thermally stable metal carbonate among the options is **A) Magnesium carbonate (MgCO₃)**.

### Final Answer:
**A) Magnesium carbonate (MgCO₃)** is the most thermally stable metal carbonate.