In physics and chemistry, a mass defect refers to the difference in mass between an atom and the sum of the masses of the protons, neutrons, and electrons of the atom.
This mass is typically associated with the binding energy between nucleons. The "missing" mass is the energy released by the formation of the atomic nucleus. Einstein's formula, E = mc2, may be applied to calculate the binding energy of a nucleus. According to the formula, when energy increases, mass and inertia increase. Removing energy reduces mass.
Mass Defect Example
For example, a helium atom containing two protons and two neutrons (four nucleons) has a mass about 0.8 percent lower than the total mass of four hydrogen nuclei, which each contain one nucleon.