11.1.2—Trend in the bond strength of the halogen

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.