Which of the following is predicted to be a gas at room temperature?

• A. Fluorine
• B. Bromine
• C. Astatine
• D. Iodine

Answer: (A)

The main idea is how the state of a substance at room temperature follows its boiling point, and how this trend appears in the halogens. Lighter halogens have very low boiling points, so they tend to be gases at room temperature, while heavier ones have higher boiling points and become liquids or solids.

Fluorine has an extremely low boiling point (around -188°C), so at room temperature it exists as a gas. Bromine has a boiling point well above room temperature (about 59°C), which is why it is a liquid in everyday conditions. Iodine boils at about 184°C, so it is a solid at room temperature. Astatine is predicted to be a solid as well, due to its large mass and stronger intermolecular forces, despite being radioactive.

Thus, fluorine is the gas at room temperature.