The Mass Defect Actually, the mass of a proton is 1.00728 amu; a neutron is 1.00866 amu. The standard is that one atom of carbon 12, the isotope of carbon with 6 protons and 6 neutrons, has a mass of exactly 12 amu. Um...if you add up 6 protons and 6 neutrons, you get more than 12 amu. Good point. The mass of 6 protons and 6 neutrons is 12.0956 amu, to be precise--but the mass of a carbon nucleus is less than the sum of its parts. That's impossible! Not only is it possible, it's essential to the universe as we know it. In the section on nuclear physics, I'll discuss the explanation and the consequences of the mysterious missing mass.

>