What is the bond angle around the carbon atoms in alkenes due to the trigonal planar shape?

The bond angle around the carbon atoms in alkenes is 120 degrees because alkenes have a trigonal planar shape around the carbon involved in the double bond. This geometry arises from the sp² hybridization of the carbon atoms, where one s orbital and two p orbitals combine to form three sp² hybrid orbitals. These orbitals are arranged in a plane with angles of 120 degrees between them to minimize electron pair repulsion according to VSEPR theory.

In the case of alkenes, the double bond consists of one sigma bond and one pi bond, and the double bond affects the geometry by enforcing a planar conformation among the carbon atoms and the associated substituents. As a result, the bond angles around each carbon in the double bond are approximately 120 degrees, consistent with the trigonal planar arrangement. This characteristic distinguishes the bonding and shapes of molecules in alkenes from those in other types of hydrocarbons, such as alkanes with tetrahedral geometry (which have bond angles of about 109.5 degrees) or linear structures found in alkynes (with bond angles of 180 degrees).