For CH3CHO, describe the hybridization of each carbon to CH3 and CHO
The methyl group is attached to 4 things, so it needs 4 orbitals – one s and 3 p -> sp^3
The carbonyl carbon is bonded to 3 things (an O, an H and the methyl group), so it needs 3 orbitals – one si 2 p –> sp^2.
Where did the other p go? To make the pi bond (C=O).
The first C atom is sp3 and the carbonyl carbon atom is sp2.