Carbonyl group:

Both aldehydes and ketones consist of a carbonyl group (C=O).  The substituents linked to the carbonyl group ascertain whether it is an aldehyde or a ketone, and whether it is aliphatic / aromatic.

The carbonyl group's geometry is planar along with bond angles of 120°. The carbon and oxygen atoms of the carbonyl group are sp² hybridized and the double bond among the atoms is build up of a strong σ bond and a weaker π bond. The carbonyl bond is shorter as compared to a C-O single bond (1.22 Å vs. 1.43 Å) and is as well stronger because two bonds are present as opposed to one (732 kJ mol^-1  vs. 385 kJ mol^-1). The carbonyl group is much more reactive than a C-O single bond because of the comparatively weak π bond.

The carbonyl group is polarized like that the oxygen is slightly negative and the carbon is slightly positive. Both the polarity of the carbonyl group and the existence of the weak π bond describe much of the chemistry and the physical properties of aldehydes and ketones. The polarity of the bond as well means that the carbonyl group has a dipole moment.

Figure: Geometry of the carbonyl group