High Quality Content by WIKIPEDIA articles! A halonium ion in organic chemistry is any onium compound (ion) containing a halogen atom carrying a positive charge. This cation has the general structure R-X+-R where X is any halogen and R any organic residue and this structure can be cyclic or an open chain molecular structure. Halonium ions formed from fluorine, chlorine, bromine, and iodine are called fluoronium, chloronium, bromonium, and iodonium, respectively. The simplest halonium ions are of the structure H-X+-H (X = F, Cl, Br, I). Halonium ions often have a three-atom cyclic structure resulting from the formal addition of a halogenium ion X+ to a C=C double bond.