The atom is electrophilic at this time and is attacked by the pi electrons of the alkene double bond. It forms for an instant a single sigma bond to "both" of the carbon atoms involved. The bonding of bromine is special in this intermediate, due to its relatively large size compared to carbon, the bromide ion is capable of interacting with both carbons which once shared the π-bond, making a three-membered ring. The bromide ion acquires a positive formal charge. At this moment the halogen ion is called a "bromonium ion" or "chloronium ion", respectively.