Physical properties of the Group 17 elements — common mistakes

When explaining boiling point trends for non-polar molecules, you must use the precise term 'instantaneous dipole-induced dipole forces' and link their strength directly to the total number of electrons in the molecule. For the F-F bond anomaly, the key phrase is 'lone pair-lone pair repulsion' due to the small atomic size.

Astatine is highly radioactive and has no stable isotopes. Its most stable isotope has a half-life of only about 8 hours, which makes it extremely difficult and dangerous to study its physical properties in the quantities needed. Its properties are generally extrapolated from the trends of the other halogens.

As non-polar molecules, halogens have low solubility in a polar solvent like water. They are much more soluble in non-polar organic solvents, such as hexane or tetrachloromethane. Note that some halogens, like chlorine, do react with water to a limited extent.

Reactivity depends on both the energy needed to break bonds (activation energy) and the energy released when forming new bonds. While the F-F bond is weak and easy to break (low activation energy), fluorine is the most electronegative element and forms exceptionally strong bonds with other elements. The very large amount of energy released when these new, strong bonds are formed makes the overall reaction highly exothermic and thermodynamically favourable.