Among the carbonates of alkali metals which one has highest thermal stability ?

Options

A. `Cs_(2)CO_(3)`
B. `Rb_(2)CO_(3)`
C. `K_(2)CO_(3)`
D. `Na_(2)CO_(3)`

Explanation

To determine which alkali metal carbonate has the highest thermal stability, we can follow these steps:

### Step 1: Understand the Concept of Thermal Stability
Thermal stability refers to the ability of a compound to withstand heat without decomposing. In the case of carbonates, this means how well they can resist breaking down into their constituent oxides and carbon dioxide when heated.

**Hint:** Remember that thermal stability can be influenced by the nature of the metal ion in the carbonate.

### Step 2: Analyze the Alkali Metal Carbonates
The alkali metal carbonates include:
- Lithium carbonate (Li2CO3)
- Sodium carbonate (Na2CO3)
- Potassium carbonate (K2CO3)
- Rubidium carbonate (Rb2CO3)
- Cesium carbonate (Cs2CO3)

**Hint:** Consider the position of these metals in the periodic table and how their properties change as you move down the group.

### Step 3: Consider the Size and Charge of the Alkali Metals
As we move down the group from lithium to cesium, the atomic size increases, and the charge density of the metal ions decreases. This affects the stability of the carbonate.

**Hint:** Larger metal ions typically form weaker bonds with carbonate ions, which can lead to lower thermal stability.

### Step 4: Evaluate the Hydration Energy
Hydration energy decreases as we go down the group. This means that the smaller alkali metals (like Li) have higher hydration energies compared to larger ones (like Cs). Higher hydration energy can contribute to greater stability in some cases.

**Hint:** Think about how hydration energy relates to the stability of the metal-carbonate bond.

### Step 5: Identify the Role of Electropositivity
Cesium (Cs) is the most electropositive of the alkali metals. This high electropositivity leads to a weaker bond between cesium and the carbonate ion, while simultaneously strengthening the bond between oxygen and carbon in the carbonate.

**Hint:** Consider how the electropositive nature of an element can influence the strength of its bonds in compounds.

### Step 6: Conclusion
Based on the analysis, cesium carbonate (Cs2CO3) has the highest thermal stability among the alkali metal carbonates because the bond between cesium and oxygen is weaker, allowing the bond between oxygen and carbon to be stronger, thus making it more stable upon heating.

**Final Answer:** The alkali metal carbonate with the highest thermal stability is **cesium carbonate (Cs2CO3)**.