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 problem, we need to identify a gaseous compound of nitrogen and oxygen that is paramagnetic, loses its paramagnetism when cooled below 0°C, and has an increase in molecular mass upon cooling.

### Step-by-Step Solution:

1. **Understanding Paramagnetism**:
- A substance is paramagnetic if it has unpaired electrons. This is typically seen in certain molecular structures where the electron configuration allows for unpaired electrons.

2. **Identifying Possible Compounds**:
- The compounds of nitrogen and oxygen include N₂O (nitrous oxide), NO (nitric oxide), NO₂ (nitrogen dioxide), and N₂O₄ (dinitrogen tetroxide).
- Among these, NO is known to be paramagnetic due to the presence of one unpaired electron.

3. **Behavior Upon Cooling**:
- The question states that when the compound is cooled below 0°C, its molecular mass increases and it loses its paramagnetism. This suggests that the gaseous compound may dimerize or undergo a phase change that results in paired electrons.
- NO, when cooled, can dimerize to form N₂O₂ (a dimer of nitric oxide), which is diamagnetic (all electrons are paired).

4. **Behavior Upon Heating**:
- When the compound is heated, it reverts back to its original paramagnetic state, indicating that the dimer breaks apart into the original gaseous form (NO), which is paramagnetic.

5. **Conclusion**:
- The compound that fits all these criteria is Nitric Oxide (NO). It is paramagnetic, dimerizes upon cooling to form a non-paramagnetic species, and returns to its paramagnetic state upon heating.

### Final Answer:
The compound is Nitric Oxide (NO).