Physical Properties of Group 7

The melting and boiling points of halogens increase as you move down the group, which corresponds to a decrease in volatility.

Halogens are simple molecular structures held together by weak van der Waals' forces (specifically instantaneous dipole-induced dipole forces) between the diatomic molecules.

As the size of the molecule increases down the group, the number of electrons increases, leading to larger electron clouds that are more easily polarized.

Stronger temporary dipoles result in stronger van der Waals' forces, requiring more thermal energy to overcome and separate the molecules during melting or boiling.

Bond enthalpy is the energy required to break one mole of covalent bonds in the gaseous state; it generally decreases down the group.

As the atoms get larger, the atomic radius increases, meaning the shared pair of electrons in the covalent bond is further from the nuclei.

This increased distance results in a weaker electrostatic attraction between the nuclei and the bonding pair, making the bond easier to break.

Fluorine ($F_2$) is an exception to the trend, possessing a lower bond enthalpy than chlorine and bromine despite its small atomic size.

Because the fluorine atom is so small, the lone pairs of electrons on the two bonded atoms are very close together, creating significant inter-electron repulsion.

This repulsion counteracts the attraction between the nuclei and the bonding pair, effectively weakening the $F-F$ bond compared to $Cl-Cl$.

Always distinguish between intermolecular forces (van der Waals' between molecules) and intramolecular bonds (covalent bonds within molecules). Boiling points depend on the former, while bond enthalpy depends on the latter.

When explaining boiling point trends, you must mention the number of electrons; more electrons lead to more polarization and stronger van der Waals' forces.

If asked about the reactivity of halogens, remember that physical properties like boiling point do not dictate chemical reactivity, though they are often discussed together.

Common mistake: Stating that covalent bonds break when a halogen boils. Only the weak van der Waals' forces between molecules are overcome.