Which among the following carbonates is thermally least stable?

Options

A. `K_2CO_3`
B. `Li_2CO_3`
C. `Na_2CO_3`
D. `Rb_2CO_3`

Answer

To determine which carbonate is thermally least stable among the given options, we need to consider the thermal stability of carbonates of alkali metals.

### Step-by-Step Solution:

1. **Understand Thermal Stability of Carbonates**:
- The thermal stability of carbonates generally increases as we move down the group in the periodic table. This means that the carbonates of heavier alkali metals are more stable than those of lighter alkali metals.

2. **Identify the Alkali Metals**:
- The alkali metals in the periodic table include Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs).

3. **Order of Stability**:
- As we go down the group from Lithium to Cesium, the thermal stability of their carbonates increases. Therefore, the order of stability is:
- Lithium Carbonate (Li2CO3) < Sodium Carbonate (Na2CO3) < Potassium Carbonate (K2CO3) < Rubidium Carbonate (Rb2CO3) < Cesium Carbonate (Cs2CO3)

4. **Conclusion**:
- Since Lithium is the lightest alkali metal, its carbonate (Li2CO3) is the least thermally stable among the alkali metal carbonates. Thus, the answer to the question is Lithium Carbonate.

### Final Answer:
- The thermally least stable carbonate is **Lithium Carbonate (Li2CO3)**.