A bromonium ion, a type of halonium ion, is a three-membered bridged cation formed when a halogen adds to a carbon–carbon double bond (C=C), the halogen straddling both carbons. It matters because its bridged structure blocks one face of the molecule, forcing the incoming nucleophile to attack the opposite face; this is what makes alkene halogenation an anti addition. It thus contrasts with an open carbenium ion, which would allow attack from either face.

溴鎓离子是卤鎓离子的一种,是卤素加到碳碳双键(C=C)上时形成的三元桥连阳离子,卤素跨接在两个碳之上。它之所以重要,是因为其桥连结构封住了分子的一面,迫使进入的亲核试剂从相反一面进攻;这正是烯烃卤化为反式加成的原因。因此,它与开放型碳正离子形成对照,后者允许从任一面进攻。