Explain the mechanism of a nucleophilic attack on the carbonyl group of
an aldehyde or a ketone.

a nucleophile is any element containing negative charge.so in a aldehyde or ketone due the high electronegativity of oxygen,partial positive charge resides on the carbon atom attached to the oxygen.therefore the nucleophile will attack the carbon containing positive charge .this results in the bond breakage of one bond between the carbon and oxygen{there is a double bond between carbon and oxygen}.negative charge is developed on the oxygen atom and the nucleophile neutralizes the carbon attached to the oxygen