The key difference between electrophilic addition reactions in alkenes and nucleophilic addition reactions in aldehydes and ketones lies in the nature of the attacking species. In electrophilic addition to alkenes, electrophiles (electron-deficient species) react with the alkene’s π electrons. In contrast, in nucleophilic addition to aldehydes and ketones, nucleophiles (electron-rich species) attack the electrophilic carbon of the carbonyl group. This carbon is partially positive due to the electronegativity difference between carbon and oxygen. Nucleophilic addition leads to the formation of a new bond between the nucleophile and the carbonyl carbon, resulting in the addition of the nucleophile to the carbonyl compound.