This “missing” mass is the mass defect, which has been converted into the binding energy that holds the nucleus together according to Einstein’s mass-energy equivalence equation, E = mc 2 . Although protons repel each other, the nucleus is held tightly together by a short-range, but very strong, force called the strong nuclear force.

An atomic nucleus consists of protons and neutrons, collectively called nucleons. A nucleus has less mass than the total mass of its constituent nucleons.

nuclear chemistry — study of the structure of atomic nuclei and processes that change nuclear structure. mass defect — difference between the mass of an atom and the summed mass of its constituent subatomic particles (or the mass “lost” when nucleons are brought together to form a nucleus). magic number — nuclei with specific numbers of nucleons that are within the band of stability. mass-energy equivalence equation — Albert Einstein’s relationship showing that mass and energy are equivalent.