Alkenes have a pi bond waiting to break — drag in a reagent and watch both atoms add across the two carbons, one molecule in, one product out. Alkanes do not invite anything: force bromine onto ethane and you need ultraviolet light, a hydrogen gets swapped, and hydrogen bromide leaves. Drag the slider to control the reagent approaching the molecule.
Hydrogen addition across ethene needs a nickel or platinum catalyst to break the H–H bond. The pi bond opens and each hydrogen forms a new single bond to a carbon — ethene becomes ethane. Every atom that started separate ends up in the product molecule. This is catalytic hydrogenation, the reaction that converts vegetable oils to margarine by saturating the carbon chains.
The double bond is the reactive site: the pi bond breaks and two new single bonds form across it, enabling four classic additions — hydrogen, halogens, hydrogen halides, and water — each conserving every atom into one product molecule.