Alkenes belong to the compound class of unsaturated hydrocarbons. Unlike alkanes in which C atoms are bound to four neighboring atoms, alkenes contain C atoms with only three neighboring atoms. In order to attain the valence of four, a C atom must share a double bond with a neighboring atom.
A hydrocarbon containing more than one double bond is called a diene, triene, polyene, etc.. Depending on how the double bonds are arranged relatively to each other, they are listed as either cumulated, or isolated.
The simplest alkene is ethene which frequently is listed with its trivial name ethylene.
Ethene is completely planar, i.e. all atoms are arranged on one level. The double bond is stronger and shorter (133 ) than a single bond (153 ). The bonding angles with ca. 120° (triangular) are larger than those in alkanes (~110°, tetrahedral).