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The difference between the mass of an atom and the sum of the masses of its individual components in the free (unbound) state. The mass of an atom is always less than the total mass of its constituent particles; this means, according to Albert Einstein's well-known formula, that an energy of E = mc2 has been released in the process of combination, where m is the difference between the total mass of the constituent particles and the mass of the atom, and c is the velocity of light. The mass defect, when expressed in energy units, is called the binding energy, a term which is perhaps more commonly used. See Nuclear binding energy
the difference between the mass of the atom of a particular isotope, expressed in atomic mass units, and the mass number, which is equal to the number of nucleons in the nucleus of that isotope. The mass defect is associated with the binding energy of the nucleons in the nucleus; it characterizes the stability of the particular nucleus. Sometimes the mass defect related to a single nucleon is used; this is called the packing fraction.