What type of reaction occurs when an alkene's double bond breaks down and atoms are added across it?

An addition reaction occurs when the double bond of an alkene breaks and new atoms are added across the bond. Alkenes contain carbon-carbon double bonds, which are reactive sites in these molecules. During an addition reaction, a reagent or another molecule interacts with the alkene, and the double bond opens up to allow for the addition of atoms or groups. This process typically results in the formation of a saturated compound where the alkene was originally present.

In the case of an alkene, the addition can happen in several ways, such as hydrogenation (adding hydrogen), halogenation (adding halogens), or hydrohalogenation (adding hydrogen halides). The key characteristic of an addition reaction is that it increases the number of single bonds in the molecule while eliminating the double bond, thus transforming the unsaturated alkene into a more saturated structure.

This reaction is fundamental in organic chemistry, as it plays a significant role in the synthesis of various organic compounds. The other types of reactions mentioned, such as substitution and elimination, involve different mechanisms that do not pertain specifically to the addition of atoms across double bonds.