Bonding:

The bonds in the carbonyl C = O group are build up of a strong σ bond and a much weaker π bond shown in figure. Because oxygen is much more electronegative than carbon, the carbonyl group is polarized like that the oxygen is slightly negative and the carbon is slightly positive. The meaning of this is that oxygen can work as a nucleophilic center and carbon can work as an electrophilic center.

Figure:   Bonding and properties.