Which of the gas is the most unsuitable for filling up weather observation balloons?

Options

A. Helium
B. Neon
C. Hydrogen
D. Argon

Explanation

To determine which gas is the most unsuitable for filling up weather observation balloons, we need to analyze the properties of the gases listed in the options. Weather observation balloons are designed to rise into the atmosphere, so the gas used must be lighter than air.

### Step-by-Step Solution:

1. **Understand the Purpose of Weather Observation Balloons**:
- Weather observation balloons are used to collect data about the atmosphere, and they need to rise into the air to do so.

2. **Identify the Requirement for the Gas**:
- The gas used to fill these balloons must be lighter than air. If the gas is heavier than air, the balloon will not rise.

3. **Analyze the Options**:
- **Option 1: Helium** - Helium is a light gas (atomic number 2) and is commonly used in balloons. It is lighter than air, making it suitable.
- **Option 2: Neon** - Neon is also a light gas (atomic number 10) and is lighter than air, thus it is suitable for use in balloons.
- **Option 3: Hydrogen (H2)** - Hydrogen is the lightest gas (molecular weight of about 2) and is significantly lighter than air, making it suitable for weather balloons.
- **Option 4: Argon** - Argon is a heavier gas (atomic number 18) and is denser than air. Therefore, it is unsuitable for filling weather observation balloons.

4. **Conclusion**:
- Based on the analysis, argon is the gas that is most unsuitable for filling up weather observation balloons because it is heavier than air and will not allow the balloon to rise.

### Final Answer:
The most unsuitable gas for filling up weather observation balloons is **argon**.