bromine

Meaning: Bromine is a halogen with seven valence electrons. It exists as Br₂, a volatile reddish-brown liquid at room temperature and 1 atm. Mercury is the other element that is liquid under these conditions. Down group 17 the electron cloud becomes larger and more polarizable, so London dispersion forces increase: F₂ and Cl₂ are gases, Br₂ is a liquid, and I₂ is a solid. Electronegativity and oxidizing strength decrease down the group, so Br₂ is a weaker oxidizer than Cl₂ but a stronger one than I₂. Br₂ is reduced to bromide, Br⁻, which has the krypton electron configuration. Br⁻ is a weaker base and a better leaving group than Cl⁻.

On the MCAT: Expect rankings of boiling point, atomic radius, electronegativity or oxidizing strength among the halogens. A displacement question may ask whether Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ proceeds. It does, because Cl₂ is the stronger oxidizer. In organic passages, Br₂ adds across an alkene by anti addition through a bromonium ion, and the loss of its brown color signals unsaturation. Radical bromination is more selective than chlorination.

Common confusion: Students assume Br₂ is a liquid because its covalent bond is strongest, or that it is more reactive than chlorine. The Br–Br bond is weaker than Cl–Cl. Br₂ is a liquid because of stronger intermolecular dispersion forces, and halogen reactivity as an oxidizer falls going down the group.