Carbonium ion, any member of a class of organic molecules with positive charges localized at a carbon atom.The carbon having positive charge is called carbocation.Carbocations are sp2 hybridized because,
With three bond pairs and zero lone pairs it has trigonal planar geometry by VSEPR theory, and trigonal planar is characteristic of sp2 hybridisation. ... A carbocation has only 3 electrons and it has to form 3 sigma bonds with its substituents. Therefore, it needs 3 hybridised orbitals, so it is sp2 hybridised.