Animation of the bonding in a molecule of ethene (C2H4). Initially there are two carbon atoms, each with full 1s and 2s electron orbitals (spheres), and one electron each in two 2p orbitals (green and orange). These then undergo sp2 hybridisation, in which a 2s electron is promoted to the third vacant 2p orbital (red). Two 2p orbitals and the 2s orbital then hybridise, merging to form three sp2 orbitals (blue) arranged in a trigonal planar (flat triangle) geometry. One 2p orbital (blue, above and below atoms) remains unhybridised. Four hydrogen atoms then form covalent bonds (blue) with two sp2 hybrid orbitals on each carbon, while the third sp2 hybrid forms a bond (centre) with its equivalent on the other carbon. These are called covalent s bonds. The formation of the carbon-carbon s bond brings the carbon atoms closer together, so the remaining 2p orbitals are very close. The adjacent 2p orbitals on the carbons then form a different type of bond, called a pi bond (blue arcs), above and below the carbon-carbon s bond. The pi bond in addition to the s bond forms what is known as a double bond between the carbon atoms. The final view shows the ethene molecule from the side, showing its planar geometry with the overlying pi bond.