Which halogen is a gas at room temperature and is pale yellow in color?

Analysis

The question asks us to identify a specific halogen based on two physical properties: its state of matter at room temperature (gas) and its characteristic color (pale yellow).

Concept Explanation

The halogens are found in Group 7 of the Periodic Table. As you move down the group, the physical properties change systematically due to increasing molecular size and stronger Van der Waals forces. Here is a quick breakdown of their physical states and colors at room temperature:

• Fluorine ($F_2$): A pale yellow gas.
• Chlorine ($Cl_2$): A greenish-yellow gas.
• Bromine ($Br_2$): A reddish-brown liquid.
• Iodine ($I_2$): A dark grey/black solid (which sublimes into a purple vapor).

Step-by-Step Solution

1. Evaluate the state: We are looking for a gas. Fluorine and Chlorine are both gases at room temperature. Bromine is a liquid, and Iodine is a solid. This narrows our options to A and B.
2. Evaluate the color: • Chlorine is described as greenish-yellow. • Fluorine is described as pale yellow.
3. Conclusion: Comparing the description in the question ("pale yellow") with the properties of the halogens, Fluorine is the only one that fits the description perfectly.

Correct Answer

The correct option is B.

Exam Tip

To remember the colors of the halogens, use the mnemonic "F-C-B-I" and visualize the transition from light to dark:

• Fluorine: Pale yellow (Lightest)
• Chlorine: Greenish-yellow
• Bromine: Reddish-brown
• Iodine: Dark grey/purple (Darkest)