Iodine is a halogen (Group VII of the Periodic Table). With atomic number 53 (electronic structure: [Kr]4d¹⁰5s²5p⁵) and atomic weight 126.90, it is the heaviest halogen apart from the radioactive element astatine. Known since about 1811, iodine is named from the Greek word for ‘violet-like’ – a reference to the color of gaseous iodine. It is widespread in trace concern traditions throughout all of Earth's geochemical reservoirs.

The elemental form, I₂, is a black crystalline solid with a metallic lustre that sublimes readily to give a violet vapor. It is reactive, partaking in both reduction and oxidation reactions, as well as iodinating organic matter. Like fluorine, chlorine and bromine, iodine accepts an electron to reach a stable inert gas configuration, but the iodide anion (I⁻) is appreciably larger (215 pm radius), more polarizable, less electronegative, and has a smaller electron affinity than the other halides. The last property means that iodide is more easily re-oxidized to...