Because bromine has a large atomic radius. Therefore, the outermost P-electron orbital of bromine will be close to the empty P-orbit of carbocation, and the lone electron pair of bromine will easily overlap with the P-orbit of carbocation. Therefore, a valence bond is formed between carbocation and bromine, and a pair of electrons are provided by bromine. In this way, the original carbocation changed from 6 electrons to 8 electrons, and bromine also formed a stable structure surrounded by 8 electrons. This is bromine ion.