Molecules containing bonds of orders higher than one (i.e., double and triple bonds) contain piπ bonds. For example, each molecule of ethene "H"_2 "C" = "C" "H"_2H2C=CH2 contains a carbon-carbon "C"="C"C=C double bond.
Each neutral carbon atom contains four valence electrons. The atom undergoes a hybridization process that would result in three sp^2sp2 orbitals and one 2p2p orbital before bonding to another carbon atom of an identical configuration and forming a carbon-carbon double bond.
![Carbon valence shell before and after hybridization, created with Google Drawings]()
As seen in the diagram, the 2p2p orbital that is left half-filled and not hybridized is capable of overlapping with an equally half-filled and unhybridized 2p2p orbital from another carbon atom.
The two 2p2p orbitals overlap in parallel ("sideway") to the bonding axis between the two nuclei and the sigma bond from the overlapping of two sp^2sp2 orbitals. Note that each pp orbital contains two petal-like electron clouds such that "two"two regions of high electron density on opposite sides of the bonding axis (shown in blue in the second diagram) makes a single piπ bond.
