Which of the following processes involves the addition of atoms across the carbon-carbon double bond in alkenes?

The addition of atoms across the carbon-carbon double bond in alkenes is a fundamental characteristic of an addition reaction. In this process, alkenes, which contain a double bond, react with other substances such as halogens, hydrogen, or water. The double bond opens up to allow new atoms or groups of atoms to bond with the carbon atoms, resulting in the formation of a saturated compound, typically an alkane or an alcohol.

For example, when an alkene reacts with hydrogen (hydrogenation), the double bond is converted into a single bond, effectively "adding" hydrogen atoms to the molecule. This type of reaction is crucial in organic chemistry as it forms the basis for many synthetic pathways, helping to transform more reactive alkenes into more stable alkanes or other functional groups.

In contrast, dehydration involves the removal of water from a compound, condensation refers to the joining of two molecules with the release of a smaller molecule, often water, and substitution reactions involve the replacement of one atom or group by another. These processes do not involve the straightforward addition of atoms across a double bond, making addition reactions distinct and integral in the chemistry of alkenes.