Cracking the SAT Chemistry Subject Test

Besides intermolecular forces and temperature, molecular weight also affects
vapor pressure. The heavier the molecule, the slower its average velocity at a
given temperature, and the harder it is to vaporize. Think about the halogens.
Fluorine and chlorine are gases at room temperature, bromine is a liquid, and
iodine is a solid. They have the same valence structure and exhibit the same
types of intermolecular forces. Molecular weight accounts for their respective
phases.