mass deficiency

Mass deficiency, also known as mass defect, is a term that describes the fact that nuclei, made up of protons and neutrons, have a smaller rest mass than the sum of the rest masses of the protons and neutrons forming the nucleus. The mass difference is equal to the released binding energy. For the alpha particle with a mass of 4.00151 atomic mass units, the structure of two protons (1.00728 atomic mass units each) and two neutrons (1.00866 atomic mass units each), a mass defect of 0.03037 atomic mass units results. This corresponds to an energy (binding energy) of about 28 million electronvolts (MeV).