sp² hybridisation is exihibited by

In the formation of ethylene, each carbon atom undergoes sp² hybridisation leaving 2p_z orbital unhybridised. One of the sp² hybridised orbital of one carbon atom overlaps axially with sp² hybridised orbital of the other carbon atom to form a stable sigma bond (C-C). The remaining two sp² hybrid orbitals of both carbon overlap axially with the half filled '1s' orbital of hydrogen atoms forming four C-H sigma bonds. The unhybridised orbital (2p_z) of one carbon atom overlaps sidewise with the similar orbital of the other carbon atom to form a weak pi bond. Thus, in ethylene all the six atoms containing the sigma bonds lie in one plane, while the pi bond is at a plane perpendicular to the plane, of the six atoms.