Which of the following gas exists in the monoatomic state ?

Options

A. Carbon dioxide
B. Hellium
C. Oxygen
D. Nitrogen

Answer

To determine which of the given gases exists in a monoatomic state, we need to analyze each option based on their molecular structure:

1. **Understanding Monoatomic State**:
- A monoatomic gas consists of single atoms that are not bonded to other atoms.

2. **Analyzing Each Option**:
- **Option A: Carbon Dioxide (CO2)**
- Carbon dioxide is composed of one carbon atom and two oxygen atoms. Therefore, it is not monoatomic.
- **Option B: Helium (He)**
- Helium is a noble gas and exists as single atoms. Thus, it is monoatomic.
- **Option C: Oxygen (O2)**
- Oxygen exists as O2, which means it consists of two oxygen atoms bonded together. Hence, it is not monoatomic.
- **Option D: Nitrogen (N2)**
- Nitrogen exists as N2, indicating it consists of two nitrogen atoms bonded together. Therefore, it is not monoatomic.

3. **Conclusion**:
- Based on the analysis, the only gas that exists in the monoatomic state is **Helium (Option B)**.

### Final Answer:
**Helium (Option B) exists in the monoatomic state.**