11.1.1—Colours and the trend in volatility

The halogens exist as diatomic molecules. Down Group 17, increasing electron count strengthens van der Waals forces, so melting and boiling points rise and volatility falls.

The visible colour also becomes deeper as electron clouds become more polarisable and electronic energy gaps change. Physical-state observations should be tied to the molecular trend.

Chlorine is a yellow-green gas, bromine a red-brown liquid and iodine a dark solid at room conditions. The change reflects stronger intermolecular attraction down the group.

The colour trend is not evidence of stronger covalent bonds within X₂ alone. Volatility and boiling point mainly reflect intermolecular forces between molecules.