In the formation of CH2=CH2 each carbon atom in its excited state undergoes sp2 hybridisation by intermixing one s- orbital (2s) and two p- orbitals (2p say 2px and 2py) and reshuffling to form three sp2 orbitals. Now each carbon atom is left with one 'p' orbital (say 2pz) unhybridized. The three sp2 orbitals having one electron each get separated around the nucleus of carbon atoms at an angle of 120°. When the carbon atom is ready to form bonds one sp2 orbital of one carbon atom overlaps with another sp2 orbital of another carbon atom to form sp2-sp2 sigma bond . The remaining two sp2 orbitals of each carbon atom get overlapped by s orbitals of two hydrogen atoms containing unpaired electrons. The unhybridised pz orbitals overlap laterally to form a π bond. That means there exists a sigma bond and a pi bond between two carbon atoms in ethene molecule .
The diagram of ethene will be shown in the attachment ..
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