If the unified atomic mass unit is 1.6605 x 10-27 kg and it is exactly 1/12 the mass of carbon-12, why are the masses of protons and neutrons higher than this (1.6726 x 10-27 kg and 1.6749 x 10-27 kg respectively)? If the unified atomic mass unit really is 1/12 the mass of carbon-12 then shouldn't 12 times this value equal the combined masses of 6 protons, 6 neutrons and 6 electrons?