Among `BeCO_(3), CaCO_(3), SrCO_(3)` and `BaCO_(3)`, how many number than of these compounds are thermally more stable than `MgCO_(3)` ?

Explanation

To determine how many of the compounds \( BeCO_3, CaCO_3, SrCO_3, \) and \( BaCO_3 \) are thermally more stable than \( MgCO_3 \), we need to analyze the thermal stability of these carbonates based on their position in the periodic table.

### Step-by-Step Solution:

1. **Understanding Thermal Stability**:
- The thermal stability of carbonates generally increases as we move down the group in the periodic table. This is due to the increasing size of the cation, which leads to a decrease in the lattice energy and makes the compound more stable against thermal decomposition.

2. **Order of Stability**:
- For alkaline earth metal carbonates, the order of thermal stability is:
\[ BaCO_3 > SrCO_3 > CaCO_3 > MgCO_3 \]
- This means that barium carbonate is the most stable, followed by strontium carbonate, then calcium carbonate, and finally magnesium carbonate.

3. **Analyzing Each Compound**:
- **Beryllium Carbonate (\( BeCO_3 \))**:
- Beryllium carbonate is known to be thermally unstable due to the high polarizing power of beryllium, which destabilizes the carbonate ion. Therefore, \( BeCO_3 \) is not more stable than \( MgCO_3 \).
- **Calcium Carbonate (\( CaCO_3 \))**:
- Calcium carbonate is less stable than magnesium carbonate according to the order established above. Thus, \( CaCO_3 \) is not more stable than \( MgCO_3 \).
- **Strontium Carbonate (\( SrCO_3 \))**:
- Strontium carbonate is more stable than magnesium carbonate. Therefore, \( SrCO_3 \) is thermally more stable than \( MgCO_3 \).
- **Barium Carbonate (\( BaCO_3 \))**:
- Barium carbonate is also more stable than magnesium carbonate. Hence, \( BaCO_3 \) is thermally more stable than \( MgCO_3 \).

4. **Conclusion**:
- From the analysis, we find:
- \( BeCO_3 \): Not more stable than \( MgCO_3 \)
- \( CaCO_3 \): Not more stable than \( MgCO_3 \)
- \( SrCO_3 \): More stable than \( MgCO_3 \)
- \( BaCO_3 \): More stable than \( MgCO_3 \)

Therefore, the number of compounds that are thermally more stable than \( MgCO_3 \) is **2** (i.e., \( SrCO_3 \) and \( BaCO_3 \)).

### Final Answer:
**2**