Urea contains a carbonyl group. The more electronegative oxygen atom pulls electrons away from the carbon forming a greater electron density around the oxygen, giving the oxygen a partial negative charge and forming a polar bond. When nitric acid is presented, it ionizes. A hydrogen ion [proton] contributed by the acid is attracted to the oxygen and forms a covalent bond [electrophile H+]. The electronegative NO3− ion then is attracted to the positive hydrogen ion. This forms an ionic bond and hence the compound urea nitrate.