A gaseous compound of nitrogen and oxygen is paramagnetic in nature. When it is cooled below `0^(@)C` its molecular mass increases and paramagnetism is lost. The behaviour is reversed on heating. The compound is

Options

A. `N_(2)O_(3)`
B. `N_(2)O`
C. `NO_(2)`
D. `N_(2)O_(4)`

Answer

To solve the question regarding the gaseous compound of nitrogen and oxygen that exhibits paramagnetism and changes its properties upon cooling and heating, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Compound**:
- We need to identify a gaseous compound of nitrogen and oxygen that is known to be paramagnetic. The most common candidates are nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4).

2. **Understand Paramagnetism**:
- A compound is paramagnetic if it has unpaired electrons. NO2 has one unpaired electron in its molecular structure, which makes it paramagnetic.

3. **Behavior upon Cooling**:
- The question states that when the compound is cooled below 0°C, its molecular mass increases and paramagnetism is lost. When NO2 is cooled, it dimerizes to form N2O4, which has no unpaired electrons, thus losing its paramagnetic nature and becoming diamagnetic.

4. **Reaction Involved**:
- The reaction that occurs upon cooling can be represented as:
\[ 2 \text{NO}_2 \rightarrow \text{N}_2\text{O}_4 \]
- In this reaction, two molecules of nitrogen dioxide combine to form one molecule of dinitrogen tetroxide.

5. **Behavior upon Heating**:
- When the temperature is increased, N2O4 can dissociate back into NO2, regaining its paramagnetic properties as it reverts to a state with unpaired electrons.

6. **Conclusion**:
- Based on the above analysis, the compound in question is nitrogen dioxide (NO2).

### Final Answer:
The compound is **NO2** (Nitrogen Dioxide).

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