the mass defect

Weigh a helium-4 nucleus on an impossibly precise scale, then weigh its ingredients separately: two protons and two neutrons. The parts come out heavier than the whole. That missing slice of mass is the mass defect. It is the accounting shortfall that first told physicists, unmistakably, that binding a nucleus releases energy, because the only place the mass could have gone is into the energy that flew away when the nucleus formed.

Formally, the mass defect of a nucleus with Z protons and N neutrons is Delta m = Z m_p + N m_n - M(Z,N), the sum of free-nucleon masses minus the actual nuclear mass. It is tied to the nuclear binding energy by Einstein's relation, B = Delta m c^2. A useful bookkeeping fact: one atomic mass unit corresponds to about 931.494 MeV, so a mass defect quoted in u converts straight into a binding energy in MeV. Be careful whether you are using nuclear masses or neutral-atom masses; the latter include the atomic electrons, and their masses (and binding) must be handled consistently.

The mass defect is the tangible fingerprint of mass-energy equivalence in the laboratory. Mass spectrometry measures nuclear masses to extraordinary precision, and every stable nuclide shows a positive defect. It is the same idea behind the tiny mass loss of a battery as it discharges or the Sun as it shines, but in the nucleus the fractional loss is large enough (parts in a thousand) to measure directly and to power reactors and stars.