In aldehydes C of carbonyl group is attached to one alkyl group and one H.
If more than one carbonyl groups are present, the nomenclature of the compound is done in following way:
Structure of the Carbonyl Group
The carbonyl C is sp2 hybridised and forms three σ-bonds. Fourth electron of carbon atom remains in unhybridised p-orbital and form π bond with oxygen by overlapping with p-orbital of oxygen. Carbon and three atoms attached to it lie in the same plane and form a trigonal coplanar structure.