Mass defect
The difference between the total mass of separated nucleons and the actual mass of a nucleus: Δm = [Z × m_p + (A−Z) × m_n] − m_nucleus. This missing mass is converted to binding energy via E = Δmc². Mass defect increases with nuclear binding energy; nuclei with larger mass defects are more stable (binding energy higher).
Formula
Δm = [Zmₚ + (A−Z)mₙ] − mₙᵤcₗₑᵤₛ
Real World
When helium-4 forms from two protons and two neutrons, the mass defect of ~0.030 u corresponds to 28.3 MeV of binding energy — the same energy release that powers hydrogen bombs via fusion.
Exam Focus
Use atomic mass unit values given in the data sheet precisely; rounding nucleon masses too early causes significant errors in binding energy calculations.
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