In the molecule of formaldehyde H2CO , the central atom is carbon. The carbon atom forms two sigma bonds with two hydrogen atoms and one sigma bond with an oxygen atom, as well as a pi bond with the oxygen atom. To accommodate these bonds, the carbon atom undergoes sp2 hybridization, which involves the mixing of one s orbital and two p orbitals to form three sp2 hybrid orbitals. These sp2 hybrid orbitals are used to form sigma bonds with the hydrogen and oxygen atoms. The remaining unhybridized p orbital on the carbon atom forms a pi bond with the oxygen atom.