mass defect

Meaning: A nucleus has less mass than the sum of its free protons and neutrons. When nucleons bind, energy is released, and the system loses a matching amount of mass. That difference is the mass defect. Multiplied by c², it gives the binding energy, the energy needed to pull the nucleus apart into free nucleons. Binding energy per nucleon is typically highest near iron-56, so fusing light nuclei or splitting very heavy nuclei usually releases energy.

Equation: Δm = Z·mp + (A − Z)·mn − mnucleus, and Eb = Δm·c². Here Z is the atomic number, A is the mass number, mp and mn are the proton and neutron masses (kg), mnucleus is the measured nuclear mass (kg), Δm is the mass defect (kg), c = 3.00 × 10⁸ m/s, and Eb is the binding energy (J). A handy conversion is that 1 u corresponds to about 931.5 MeV.

On the MCAT: A passage gives masses in atomic mass units. You find Δm, then convert to energy with 931 MeV per u or with E = mc². You may also compare binding energy per nucleon on a curve to decide which nucleus is more stable, or find the energy released in fission, fusion or decay from the mass difference between reactants and products.

Common confusion: Students think the mass is destroyed or that a bigger defect means a weaker nucleus. The mass leaves as released energy, and a bigger binding energy means the nucleus is more tightly bound.