Shown here is the intermediate product that forms when ethene reacts with bromine, Br_2. This so-called bromonium ion is formed when the weakly nucleophilic double bond of ethene reacts with Br₂, generating the bromide anion (Br-). In the bromonium ion, the bromine atom is shared between the two carbons. The resulting delocalized charge is represented by the blue area in the electrostatic potential diagram. Because the bromine atom takes up so much room on top on the molecule, an attacking bromide ion will approach underneath, that is the side opposite the bromine atom of the bromonium ion. As a result, the bromine atoms end up on opposite sides of what used to be the carbon-carbon bond. This term is called anti-addition. As a result of these considerations, constrained systems such as cycloalkenes will give only trans dibromo cycloalkane products.