triiodide ion

In chemistry, triiodide usually refers to the triiodide ion, . This anion, one of the polyhalogen ions, is composed of three iodine atoms. It is formed by combining aqueous solutions of iodide salts and iodine. Some salts of the anion have been isolated, including thallium(I) triiodide (Tl+[I3]−) and ammonium triiodide ([NH4]+[I3]−). Triiodide is observed to be a red colour in solution.

==Nomenclature== Other chemical compounds with "triiodide" in their name contain three separate or independent iodide centers rather than a single unit that consists of three iodine atoms. The prefix "tri" in that situation is a multiplicative prefix for iodide—a 3:1 stoichiometric ratio—rather than part of the name of the unified three-iodine structure. Examples incllude nitrogen triiodide (NI3) and phosphorus triiodide (PI3), each of which have a central atom to which each of the three iodine atoms is bonded. To clarify the structural nature of a chemical with three iodides, other naming schemes can be used. Typical options are to use tris as an alternative prefix, such as the chemical thallium tris(iodide), where there three "iodide" units, as distinct from the chemical thallium(I) triiodide, where thallium is oxidation state +1 and therefore "triiodide" is the single monovalent unit.