 A second addition reaction is called halogenation. As it sounds, and let's start again with our ethene, and this time we're going to add a halogen. So let's add chlorine. If we add chlorine gas to this particular substance, then again the chlorine is going to add into that spot where the double bond is, and one chlorine atom is going to go either side of that bond. So we'll come back to a single bond. Once again we have a hydrogen, hydrogen, hydrogen, hydrogen, but this time we have chlorines adding across the top. So this time our product would be a 1, 2, di-chloroethane, a 1, 2 di-chloroethane where this time it's our halogen that's added across the double bond. Then two reactants and just the one product. Now the same thing would happen if we added fluorine, or bromine, or iodine, all of which are diatomic molecules, and all of which will add across the double bond in order to produce a 1, 2 dihalo, in this case ethane.