Which of the following is monoatomic?

Options

A. Hydrogen
B. Chlorine
C. Helium
D. Nitrogen

Explanation

To determine which of the given options is monoatomic, we first need to understand what monoatomic means. Monoatomic refers to a substance that consists of single atoms, meaning it has an atomicity of one.

### Step-by-Step Solution:

1. **Understand Monoatomic Definition**:
- Monoatomic substances are made up of individual atoms. Noble gases are common examples of monoatomic elements.

2. **Identify the Options**:
- The options provided are:
1. Hydrogen (H)
2. Chlorine (Cl)
3. Helium (He)
4. Nitrogen (N)

3. **Analyze Each Option**:
- **Option 1: Hydrogen (H)**
- Hydrogen exists as diatomic molecules (H₂) in nature. Therefore, it is not monoatomic.
- **Option 2: Chlorine (Cl)**
- Chlorine also exists as diatomic molecules (Cl₂) in nature. Thus, it is not monoatomic.
- **Option 3: Helium (He)**
- Helium is a noble gas and exists as single atoms (He). Therefore, it is monoatomic.
- **Option 4: Nitrogen (N)**
- Nitrogen exists as diatomic molecules (N₂) in nature. Hence, it is not monoatomic.

4. **Conclusion**:
- The only element among the options that is monoatomic is **Helium (He)**.

### Final Answer:
- The correct option is **Option 3: Helium (He)**.