Mass Defect
The difference between the total mass of an atom's individual free nucleons and the actual smaller mass of the bound atomic nucleus.
Governing formula
Δm = Z m_p + (A - Z) m_n - M_nucleus
SI unit
u (Atomic Mass Units) or MeV/c²
In depth
Mass defect is a direct consequence of Einstein's mass-energy equivalence E = Δm c². The missing mass represents energy released when nucleons bind together via the strong force.
Examples in the real world
A Helium-4 nucleus having a mass defect Δm ≈ 0.03037 u, corresponding to a binding energy of 28.3 MeV.