I read that a C12 atom has an atomic mass unit of 12 amu's. A C12 atom has 6 protons, 6 neutrons, and 6 electrons. If the mass of a proton is 1.00728 amu, the mass of a neutron is 1.00867 amu, and the mass of an electron is .000549, how can it's total amu be 12? 6*1.00728+6*1.00867+6*.000549= 12.098994 amu How did they determine that a C12 atom is 12 amu's? Are they giving an approximate answer?