Glossary · Vật lý hạt nhân
Nuclear binding energy
The energy released in forming a nucleus from its nucleons. It equals the mass defect times c², because a nucleus has less mass than the sum of the nucleons that make it up.
Weigh a nucleus, then add up the separate masses of the protons and neutrons inside it, and the two numbers differ: the nucleus is LIGHTER. The difference is the mass defect.
That mass did not vanish; it was released as energy when the nucleus formed, following E = Δm·c². This is the nuclear binding energy, and it is equally the energy that must be supplied to pull the nucleus back apart into free nucleons.
For nuclei with mass number A greater than 8, the binding energy is roughly proportional to the total nucleon count A. Divide by A and you get the binding energy PER nucleon — a quantity that ranges from 6 to 10 MeV, averaging about 8 MeV. Its curve rises at low A, peaks very near iron (Fe, A = 56) and then tapers off at high A; that peak is why the iron nucleus is the most stable nucleus in nature.
The shape of that curve is what decides which reactions release energy: fusion of nuclei with mass numbers much less than that of iron, and fission of nuclei with mass numbers greater than that of iron, are both exothermic.