核結合エネルギー

Nuclear stability is best quantified in terms of nuclear binding energy. Consider the helium-4 atom, which has two each of protons, neutrons, and electrons. The sum of the known masses of these particles is greater than the measured mass of neutral helium-4 by 0.0305 atomic mass units. The difference between the calculated and experimentally measured atomic masses is called the mass defect. The large amount of energy released during the formation of helium-4 is the reason for this difference. Einstein’s mass–energy equivalence helps to estimate the energy change associated with the loss in mass. Converting the mass to kilograms and solving the equation results in the base SI units for joules. It is evident that an enormous amount of energy accompanies the tiny change in mass. The energy released when the nucleons bind together is the same as the energy required to break that nucleus into its constituent protons and neutrons and is called nuclear binding energy. For helium, this is 2.74 terajoules per mole.  Dividing by Avogadro's number gives 4.55 picojoules for the nuclear binding energy per helium nucleus. This is often expressed in electronvolts as well. For helium-4, this turns out to be 28.4 megaelectronvolts per nucleus. When divided by the number of nucleons, 4, this yields the nuclear binding energy per nucleon. The plot of nuclear binding energy per nucleon versus mass number depicts the comparative stabilities of the nuclides. The elements with mass numbers ranging from 40 to 100 have the highest per-nucleon binding energy, and iron-56 has the lowest mass per nucleon.  To attain stability, heavy nuclei tend to fragment to midsize nuclei through an exothermic process called fission, whereas lighter nuclei combine through the fusion process