Which of the following is a solid at room temperature?

Analyze the question: We need the substance that exists as a solid at room temperature (about 25 °C).

Concept: The physical state of the halogens at room temperature depends on the strength of the van der Waals (London dispersion) forces between their molecules. These forces grow stronger as molecular size and mass increase down Group 17, so melting and boiling points rise down the group.

Step by step:

| Element | State at room temperature | |---|---| | Fluorine (F₂) | Pale yellow gas | | Chlorine (Cl₂) | Greenish-yellow gas | | Bromine (Br₂) | Reddish-brown liquid | | Iodine (I₂) | Dark grey/black solid (sublimes on heating to give purple vapour) |

Argon is a noble gas. It is monatomic, with very weak forces between atoms, so it is a gas.

• A. Argon: gas
• B. Bromine: liquid
• C. Chlorine: gas
• D. Fluorine: gas
• E. Iodine: solid

Tip: Remember the halogen states with "Good Guys Like Sugar": Fluorine and Chlorine are Gases, Bromine is a Liquid, Iodine (and astatine) are Solids. Also, iodine is a classic example of a substance that sublimes, which NECO and WASSCE often ask about.