Alkenes

Alkenes are very much like alkanes in that they are hydrocarbons.
Hydrocarbons contain carbon and hydrogen only.
Alkenes differ from alkanes in that they have a C=C double bond.
The presence of this C=C double bond makes the molecules unsaturated.
Ethene is the simplest of all of the alkenes:
The alkenes are named in the same way as the alkanes. Take pent-1-ene, the pent tells us that the longest chain contains five carbons, the ene suffix tells us its part of the alkene homologous series, and the number tells us where the C=C double bond is found.
All alkenes have the same empirical formula:
CH₂
This is the same as the empirical formula for the cycloalkanes; however the double bond makes alkenes react differently to cycloalkanes.
There are also cycloalkenes (for example cyclopentene), and there are also dienes, like penta-1,3-diene

Shapes of Alkenes

Ethene is a planar molecule (all of the atoms are within the same plane.
Ethene is a flat molecule, due to it being planar.
The bonds around the C=C group are arranged the same in every alkene.
This can be explained by saying that all of the electrons want to be as far away from each other as possible.
The repulsion of the electrons is minimised when the bond angle is equal to 120^o