Which of the following is least thermally stable ?

Options

A. `MgCO_3`
B. `CaCO_3`
C. `SrCO_3`
D. `BeCO_3`

Explanation

To determine which of the given carbonates is least thermally stable, we need to analyze the factors affecting the thermal stability of metal carbonates. The thermal stability of a carbonate generally depends on the size and charge of the metal cation, as well as its polarizing ability.

### Step-by-Step Solution:

1. **Understand Thermal Stability**:
- Thermal stability refers to the ability of a compound to withstand heat without decomposing. For carbonates, this means how readily they can decompose into metal oxides and carbon dioxide (CO2) when heated.

2. **Identify Factors Affecting Stability**:
- The stability of metal carbonates can be influenced by the size and charge of the metal cation. Smaller cations with higher charges have greater polarizing power, which can destabilize the carbonate ion (CO3^2-).

3. **Analyze the Cations**:
- Consider the cations in the carbonates being compared. For example, if we are comparing beryllium (Be^2+) with other alkaline earth metals like magnesium (Mg^2+) or calcium (Ca^2+):
- Beryllium (Be^2+) is smaller in size and has a higher charge density compared to magnesium and calcium.

4. **Evaluate Polarizing Ability**:
- The polarizing ability of a cation increases with its charge and decreases with its size. Beryllium, being small and having a +2 charge, has a high polarizing ability. This means it can distort the electron cloud of the carbonate ion significantly, leading to its decomposition.

5. **Determine Decomposition Products**:
- When a carbonate decomposes, it typically forms a metal oxide and carbon dioxide (CO2). The more polarizing the cation, the more likely it is to decompose the carbonate.

6. **Conclude on Stability**:
- Since beryllium carbonate (BeCO3) has a high polarizing ability due to the small size and high charge density of Be^2+, it is the least thermally stable among the carbonates being compared. It can easily decompose to form beryllium oxide (BeO) and CO2.

7. **Final Answer**:
- Therefore, the least thermally stable carbonate is **BeCO3** (beryllium carbonate).