Halonium ion

Skeletal formula of the bromonium ion made by adding a bromine atom to cyclopentene.

In chemistry, halonium is a type of ion where an atom of a halogen (fluorine, chlorine, bromine, or iodine) has two chemical bonds and a positive electric charge. In organic chemistry, they are an intermediate step in halogenation reactions that add halogen atoms to organic compounds.

Making a salt that contains halonium ions is hard. Halogen atoms do not like having a positive charge, so halonium ions are unstable and react easily with other chemicals. Salts of halonium ions have been made using weakly coordinating anions, negatively charged ions that are extremely stable.