The nucleus of an atom is composed of neutrons and protons, collectively known as nucleons. Neutrons have no electric charge, while protons are positively charged and therefore repel each other. If electrostatic repulsion were the only force acting within the nucleus, atomic nuclei containing more than one proton could not remain bound together. Gravity cannot bind the protons together because the gravitational attraction between them is about 10³⁶  times weaker than their electrostatic repulsion. The nucleus is bound together by the strong nuclear force. This force is extremely strong over very short distances but rapidly becomes negligible beyond the dimensions of an atomic nucleus. It acts between neighbouring nucleons and is largely independent of their electric charge.

Binding energy per nucleon as a function of mass number.

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Binding energy curve

To separate a nucleus completely into its individual protons and neutrons, energy must be supplied. This energy is known as the nuclear binding energy. Dividing it by the number of nucleons gives the binding energy per nucleon, usually expressed in megaelectron volts (MeV). The binding energy of the ¹H nucleus is zero because it consists of a single proton. For the ⁴He nucleus, the binding energy is about 7.1 MeV per nucleon. For light nuclei, the binding energy per nucleon generally increases with increasing mass number. It reaches a maximum of about 8.8 MeV for nuclei in the iron-nickel region and then gradually decreases, reaching about 7.6 MeV per nucleon for uranium.

One electron volt (eV) is the energy gained by an electron when it moves through an electric potential difference of one volt.

Schematic representation of the energy released in the fission of a ²³⁵U nucleus. Approximately 200 MeV of energy is released per fission.

If light nuclei combine to form a more tightly bound nucleus, the difference in binding energy is released. For example, when a deuterium nucleus and a tritium nucleus fuse, they form a helium-4 nucleus and a neutron, releasing 17.6 MeV of energy.

Mass defect of a helium-4 nucleus. The mass defect is the difference between the sum of the masses of its individual nucleons and the mass of the bound nucleus.

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Mass deficiency of a nucleus

Similarly, when a heavy nucleus such as uranium undergoes fission, the resulting nuclei have a higher binding energy per nucleon and the difference is released as energy. This corresponds to roughly 0.9 MeV per nucleon, or about 200 MeV per fission.

Controlled nuclear fusion, in which light atomic nuclei combine and release energy, is still under development as a potential source of electricity. Present-day nuclear power plants obtain energy primarily from the fission of uranium and plutonium isotopes. Thorium can also be used in a nuclear fuel cycle by converting fertile ²³²Th into fissile ²³³U. Nuclear fission converts approximately 0.1% of the rest mass of the reacting material into energy.

The energy released by the fission of a single uranium nucleus is approximately 3.2 × 10⁻¹¹ joules. That is about five orders of magnitude less than the kinetic energy of a flying mosquito.

Depiction of the structures of hydrogen and helium atoms.

Model of the stages of the fission of a uranium nucleus, releasing approximately 0.1% of its rest energy.