Most metal oxide are thermally stable at temperature upto `1000^(@)C` but the oxide of metals below hydrogen in the electrochemical series fairly. Thus `HgO and Ag_(2)O` decomposes on heating.
Q. Which of the following compound cannot be thermally decomposed even at high temperature?

Options

A. `CsHCO_(3)`
B. `Rb_(2)CO_(3)`
C. `Li_(2)CO_(3)`
D. `(NH_(4))_(2)CO_(3)`

Answer

To determine which of the given compounds cannot be thermally decomposed even at high temperatures, we will analyze the thermal stability of each compound mentioned in the question: cesium bicarbonate, rubidium carbonate, lithium carbonate, and ammonium carbonate.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- The compounds in question are:
- Cesium bicarbonate (CsHCO₃)
- Rubidium carbonate (Rb₂CO₃)
- Lithium carbonate (Li₂CO₃)
- Ammonium carbonate ((NH₄)₂CO₃)

2. **Understand Thermal Stability**:
- Generally, the thermal stability of metal carbonates increases as we move down the group in the periodic table. This means that heavier alkali metals will have more stable carbonates compared to lighter ones.

3. **Analyze Each Compound**:
- **Cesium Bicarbonate (CsHCO₃)**: Being a bicarbonate, it is less stable and can decompose upon heating.
- **Rubidium Carbonate (Rb₂CO₃)**: As rubidium is lower in the group, its carbonate is more thermally stable and is less likely to decompose at high temperatures.
- **Lithium Carbonate (Li₂CO₃)**: Lithium, being at the top of the alkali metals, has a less stable carbonate that can decompose upon heating.
- **Ammonium Carbonate ((NH₄)₂CO₃)**: Ammonium carbonate is also thermally unstable and decomposes when heated.

4. **Conclusion**:
- Among the four compounds, **Rubidium Carbonate (Rb₂CO₃)** is the only compound that cannot be thermally decomposed even at high temperatures.

### Final Answer:
Rubidium carbonate (Rb₂CO₃) cannot be thermally decomposed even at high temperatures.