一條 chem 問題...

All halogens adapt simple molecular structure with week van der Waals' force between molecules.
Down the group, there is an increase in molecular size and the strength of van der Waals' force increases as well.
Stronger intermolecular force implies a higher boiling or melting point as more energy is required to break them in the corresponding process.
Thus the order of b.p. / m.p. in increasing order is:
Fluorine, Chlorine, Bromine, Iodine
This explains why iodine is a solid, bromine is a liquid, while chlorine and fluorine are gases at room conditions.