11.2.1—Reactivity of the elements as oxidising agents

A halogen acts as an oxidising agent when X₂ gains electrons to form halide ions: X₂ + 2e⁻ → 2X⁻. Oxidising power decreases from fluorine to iodine.

Down the group, the incoming electron is attracted less strongly because the atom is larger and more shielded. This makes reduction of X₂ less favourable, even though the molecules have more electrons.

Chlorine can oxidise bromide ions: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. Bromine cannot similarly displace chloride under the same conditions.

Oxidising strength is not the same as volatility or colour. Decide it from electron-transfer evidence or displacement reactions.