Formation and structure of ethylene molecule:
In ethylene and in other organic compounds having C = C bond, 2s and two 2p orbitals of carbon atom undergoes sp2 hybridization.
At normal state the electronic configuration of carbon atom C(6) = ls2 2s2 2px1 2py1 2pz0
At excited state the electronic configuration is C*(6) = ls2 2s1 2px1 2py1 2pz1
After sp2 hybridization the electronic configuration is C*(6) = 1s2 ψ11 ψ21 ψ31 pz1
Where ψ1, ψ2, ψ3 are sp2 hybrid orbitals. Then are hybrid orbital of one carbon atom overlaps with one hybrid orbital of another carbon atom and thus form C-C sigma (σ) bond. Two other hybrid orbitals of each carbon atom overlap with 1s orbitals of two hydrogen atoms and thus form two C-H bonds with each carbon atom. Thus the main structure of ethylene is built. Due to sp2 hybridization, the bond angles are ethylene arc about 120° and all the atoms are on a plane.
Then the 2pz orbital on each carbon atom remains with one electron each. They overlap with each other sidewise to form one C-C pi-bond.