Nuclear fission
Nuclear fission splits atomic nuclei, releasing vast energy.
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The process typically releases neutrons and gamma rays, and it yields an enormous amount of energy, far exceeding that of ordinary radioactive decay. For instance, the free energy from splitting a single uranium-235 atom is roughly 100 million times greater than the energy from burning one carbon atom in air. This reaction was discovered in December 1938 by chemists Otto Hahn and Fritz Strassmann, who proved that fission had occurred; physicists Lise Meitner and Otto Robert Frisch then provided the theoretical explanation in January 1939. Frisch coined the term "fission" by analogy with biological cell division. In their second paper on the topic, Hahn and Strassmann predicted the release of additional neutrons during fission, which raised the possibility of a nuclear chain reaction if a critical mass of fissile material were assembled.
For heavy elements, fission is exothermic, releasing energy as electromagnetic radiation and as kinetic energy of the fragments, which heats the surrounding material. For energy to be produced, the total binding energy of the resulting nuclei must exceed that of the original nucleus, and the fission barrier must be overcome. Fissionable nuclides generally split when struck by fast neutrons, while fissile nuclides—such as uranium-233, uranium-235, and plutonium-239—split easily with slow (thermal) neutrons. These three isotopes can sustain a chain reaction, enabling controlled energy release in nuclear power plants or uncontrolled release in nuclear weapons. Fission is a form of nuclear transmutation, as the daughter atoms differ from the parent. Most fissions are binary, producing two fragments of comparable but slightly different sizes, typically with a mass ratio near 3 to 2. Ternary fission, yielding three charged fragments, occurs rarely, about 2 to 4 times per 1000 events. The unpredictable composition of products distinguishes fission from quantum tunneling processes like alpha decay. Spontaneous fission, discovered in 1940, occurs without neutron bombardment in very heavy, neutron-rich isotopes. Though negligible in cosmic evolution, natural nuclear fission reactors have formed under rare conditions.
- discovered_by
- Otto Hahn, Fritz Strassmann, Lise Meitner, Otto Robert Frisch
- theoretical_explanation_date
- January 1939
- field
- Nuclear physics, chemistry
- known_for
- Splitting atomic nuclei, chain reaction, energy release
Lore & Background
Physicists Lise Meitner and her nephew Otto Robert Frisch explained it theoretically in January 1939. Frisch named the process "fission" by analogy with biological fission of living cells. In their second publication on nuclear fission in February 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction by assembling a critical mass of fissile material. Most fissions are binary fissions producing two charged fragments, but occasionally ternary fission produces three positively charged fragments.
Reader's Guide
Nuclear fission is a reaction where an atomic nucleus splits into smaller nuclei, releasing neutrons, gamma rays, and a vast amount of energy—roughly one hundred million times more energy per atom than burning a carbon atom. Discovered in late 1938 by chemists Otto Hahn and Fritz Strassmann, who proved the reaction occurred, it was theoretically explained in early 1939 by physicists Lise Meitner and Otto Robert Frisch, who named it by analogy with biological cell division. In their subsequent publication, Hahn and Strassmann predicted the liberation of additional neutrons, enabling a nuclear chain reaction if a critical mass of fissile material is assembled. For heavy nuclides, fission is exothermic, releasing energy as electromagnetic radiation and kinetic energy of fragments, which heat the bulk material. The process requires overcoming a fission barrier and, for energy production, the total binding energy of the resulting elements must exceed that of the starting element. Fissionable nuclides split primarily with fast neutrons, while fissile nuclides like uranium-235, uranium-233, and plutonium-239 split easily with slow thermal neutrons. This allows nuclear power plants to operate in a delayed critical state for controllable energy release, and nuclear weapons to achieve prompt supercritical states for uncontrolled release in about a microsecond. Most fissions are binary, producing two charged fragments, though ternary fission occurs rarely. Spontaneous fission, discovered in 1940, is a radioactive decay in very high-mass-number isotopes without external neutron induction. While nuclear fusion powers stars and created the elements, fission is negligible for universal evolution, though natural nuclear fission reactors have formed under rare conditions.
Did You Know?
- The fission process often produces neutrons and gamma rays, and releases a very large amount of energy even by the energetic standards of radioactive decay.
More in Particle And Nuclear Physics 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record