11.1 Physical properties of the Group 17 elements

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.

Halogen molecules are diatomic, X₂, with a covalent bond between the two atoms. Down Group 17, atomic size and electron count increase, but the trend in X–X bond strength is not explained by molecular mass alone.

The attraction between the two nuclei and the shared pair competes with greater internuclear distance and electron–electron repulsion in larger molecules. Use the measured trend or supplied data rather than assuming every bond becomes stronger down the group.

When comparing Cl₂, Br₂ and I₂, distinguish the covalent bond inside each molecule from the van der Waals forces between molecules. The latter clearly become stronger as the electron cloud becomes more polarizable.

Higher boiling point does not prove a stronger X–X covalent bond; it mainly reflects intermolecular attraction.

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.