halogens

Meaning: Halogens have the valence configuration ns²np⁵, one electron short of an octet. They have high electronegativity and strongly exothermic electron affinities, form X⁻ ions, and exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). At room temperature F₂ and Cl₂ are gases, Br₂ is a liquid and I₂ is a solid, because London dispersion forces grow with size and polarizability. Down the group, atomic radius increases while electronegativity, ionization energy and oxidizing strength decrease.

On the MCAT: Expect trend rankings, such as electronegativity F > Cl > Br > I, or the fact that I⁻ is the largest halide. Hydrohalic acid strength is HI > HBr > HCl >> HF, because the H–X bond weakens down the group. In organic chemistry, I⁻ is the best leaving group among the halides (weakest base, most polarizable). In radical halogenation of alkanes, Br₂ is more selective than Cl₂. Chlorine also shows positive oxidation states in oxoanions, from +1 in ClO⁻ to +7 in ClO₄⁻.

Common confusion: Students assume the most electronegative halogen gives the strongest acid, so they rank HF highest. Within a group, bond strength and anion size control acidity, so HF is a weak acid and HI is a strong one. Fluorine also has only the oxidation state −1 in compounds.