Hybridization

In formaldehyde ( ), the carbon atom is double-bonded to oxygen and single-bonded to a hydrogen atom and a methyl group ( ). This requires three regions of electron density around the carbon atom. Hence, the carbon atom undergoes hybridization to form three sp² hybrid orbitals, which results in a trigonal planar shape.
The shape of the formaldehyde molecule is trigonal planar due to the hybridization of the central carbon atom.