1) Alkenes are unsaturated hydrocarbons that contain one or more carbon-carbon double bonds. The general formula for alkenes is CnH2n. 2) A carbon-carbon double bond consists of one sigma bond and one pi bond formed from the overlap of sp2 hybrid orbitals and unhybridized p orbitals. 3) The cis-trans or E-Z systems are used to describe the stereochemistry of alkene double bonds and cycloalkene rings.