Nuclear Fission Versus Nuclear Fusion

Atomic nuclei combine in nuclei fusion and break into smaller pieces in nuclear fission.
Atomic nuclei combine in nuclei fusion and break into smaller pieces in nuclear fission. MARK GARLICK / Getty Images

Nuclear fission and nuclear fusion both are nuclear phenomena that release large amounts of energy, but they are different processes which yield different products. Learn what nuclear fission and nuclear fusion are and how you can tell them apart.

Nuclear Fission

Nuclear fission takes place when an atom's nucleus splits into two or more smaller nuclei. These smaller nuclei are called fission products.

Particles (e.g., neutrons, photons, alpha particles) usually are released, too. This is an exothermic process releasing the kinetic energy of the fission products and energy in the form of gamma radiation. The reason energy is released is because the fission products are more stable (less energetic) than the parent nucleus. Fission may be considered a form of element transmutation since changing the number of protons of an element essentially changes the element from one into another. Nuclear fission may occur naturally, as in the decay of radioactive isotopes, or it can be forced to occur in a reactor or weapon.

Nuclear Fission Example

23592U + 10n → 9038Sr + 14354Xe + 310n

Nuclear Fusion

Nuclear fusion is a process in which atomic nuclei are fused together to form heavier nuclei. Extremely high temperatures (on the order of 1.5 x 107°C) can force nuclei together so the strong nuclear force can bond them.

Large amounts of energy are released when fusion occurs. It may seem counterintuitive that energy is released both when atoms split and when they merge. The reason energy is released from fusion is because the two atoms have more energy than a single atom. A lot of energy is required to force protons close enough together to overcome the repulsion between them, but at some point, the strong force that binds them overcomes the electrical repulsion.

When the nuclei are merged, the excess energy is released. Like fission, nuclear fusion can also transmute one element into another. For example, hydrogen nuclei fuse in stars to form the element helium. Fusion is also used to force together atomic nuclei to form the newest elements on the periodic table. While fusion occurs in nature, it's in stars, not on Earth. Fusion on Earth only occurs in labs and weapons.

Nuclear Fusion Examples

The reactions which take place in the sun provide an example of nuclear fusion:

11H + 21H → 32He

32He + 32He → 42He + 211H

11H + 11H → 21H + 0+1β

Distinguishing Between Fission and Fusion

Both fission and fusion release enormous amounts of energy. Both fission and fusion reactions can occur in nuclear bombs. So, how can you tell fission and fusion apart?

  • Fission breaks atomic nuclei into smaller pieces. The starting elements have a higher atomic number than that of the fission products. For example, uranium can fission to yield strontium and krypton.
  • Fusion joins atomic nuclei together. The element formed has more neutrons or more protons than that of the starting material. For example, hydrogen and hydrogen can fuse to form helium.
  • Fission occurs naturally on Earth. An example is the spontaneous fission of uranium, which only happens if enough uranium is present in a small enough volume (rarely). Fusion, on the other hand, does not occur naturally on Earth. Fusion occurs in stars.