The reaction of ethene with bromine is called an addition reaction because the bromine molecule adds across the double bond of the ethene molecule. Here's a breakdown:

* Ethene: Ethene (C₂H₄) has a carbon-carbon double bond. This double bond represents a region of high electron density, making it susceptible to attack by electrophiles.

* Bromine: Bromine (Br₂) is a diatomic molecule with a weak Br-Br bond. This bond can be easily broken, and the bromine atoms become electrophilic (electron-seeking).

* Reaction: When bromine encounters ethene, the bromine molecule breaks apart. One bromine atom attacks the electron-rich double bond in ethene, breaking the pi bond and forming a new carbon-bromine single bond. This process leaves a carbocation intermediate, which is then attacked by the other bromine atom, forming another carbon-bromine bond.

* Product: The final product is 1,2-dibromoethane (CH₂BrCH₂Br), where the two bromine atoms have added across the double bond, effectively "saturating" the double bond.

In summary:

* The double bond in ethene breaks.

* Two new single bonds are formed with the bromine atoms.

* No atoms are removed from the molecule, only added to it.

This process of adding atoms across a double or triple bond is why the reaction is called an addition reaction.