Iodine dioxide

Iodine dioxide is a binary inorganic compound of iodine and oxygen with the chemical formula IO
₂. Only stable as a dilute gas, this compound is one of many iodine oxides, and "iodine dioxide" is sometimes used to describe its formal dimer, the salt diiodine tetroxide (I₂O₄, [IO]⁺[IO₃]⁻).

It is formed, in parts-per-trillion quantities, at the marine boundary layer and believed to mediate particulate nucleation therein. Atomic iodine, originally from photolysis of algaeic diiodomethane, reacts with ozone to produce iodine monoxide, which then disproportionates to atomic iodine and iodine dioxide. At a sufficiently large concentration, the particles then combine to form small grains of diiodine tetroxide.