11.1.3—The volatility of the elements

Volatility is the tendency of a substance to enter the gas phase. For the halogens, volatility decreases down Group 17 as London dispersion forces between X₂ molecules become stronger.

More electrons make the electron cloud more polarizable. Temporary dipoles therefore induce stronger attractions, raising melting and boiling temperatures and changing the physical state at room temperature.

Chlorine is a gas, bromine a liquid and iodine a solid under ordinary conditions. This sequence is a physical-property trend, not a change from molecular to ionic bonding.

Do not explain the trend as stronger covalent bonds between halogen atoms. The key change is attraction between molecules.