Carbonyl groups are electrophilic due to the presence of a polar covalent bond between the carbon and oxygen atoms. The oxygen atom in the carbonyl group is more electronegative than the carbon atom, so it pulls electrons away from the carbon atom, creating a partial positive charge on the carbon atom. This partial positive charge makes the carbonyl group susceptible to attack by nucleophiles, which are species that have a lone pair of electrons that can be donated to another atom.

In addition, the carbonyl group is also resonance stabilized, which further increases its electrophilicity. Resonance stabilization occurs when electrons can move around a molecule to create multiple possible structures. In the case of the carbonyl group, the electrons in the pi bond between the carbon and oxygen atoms can move to form a double bond between the carbon atom and one of the oxygen atoms, and a single bond between the carbon atom and the other oxygen atom. This resonance stabilization helps to spread out the positive charge on the carbon atom, making it more stable and more likely to react with nucleophiles.