Structure and formula
Alkenes contain at least one C=C double bond. For open-chain compounds with one double bond and no other unsaturation, the general formula is CnH2n (n≥2). The suffix -ene identifies the double bond, and numbering gives it the lowest permitted locant.
Bonding and geometry
Each carbon of a simple C=C bond is approximately sp2-hybridized and trigonal planar with angles near 120°. The double bond contains one sigma bond and one pi bond. The pi bond arises from sideways overlap of parallel p orbitals and prevents free rotation without bond breaking.