Nuclear fission diagram shows a neutron being absorbed by a uranium nucleus which then becomes unstable and splits into two new atoms with release of energy and some more neutrons.
**Nuclear fission** is a kind of nuclear reaction. It is when an atom splits apart into smaller atoms. Some fission reactions give off a lot of energy, and are used in nuclear weapons and nuclear reactors. Nuclear fission was discovered in December 1938 by physicists Lise Meitner and Otto Frisch, who observed a uranium nucleus splitting in two. Although they originally thought the two tiny resulting nuclei were barium isotopes, what Meitner and Frisch truly saw was a process that would revolutionize nuclear chemistry: nuclear fission

An atom is the smallest particle which makes up a chemical element (e.g. hydrogen, oxygen, magnesium). All atoms are very small. Atoms are made of three components or particles: Protons, neutrons and electrons. The protons and neutrons are clumped together in a ball called a nucleus, at the center of every atom. The electrons orbit around the nucleus in its 'electron cloud'. Elements which have large nuclei, such as uranium and plutonium, can be made to fission. 

If a (relatively) very large atomic nucleus is hit by a slow-moving neutron, it will sometimes become unstable and break into two nuclei. When the nucleus breaks apart (or **fissions**) it releases energy, mostly as gamma rays and heat. It also causes some neutrons to be released from the nucleus.

For a few isotopes (an atom with the same amount of protons but a different amount of neutrons) such fission can release many neutrons. If those neutrons then hit other atoms, they will make the other atoms split. This can happen again and again. This is called a nuclear chain-reaction, and it can release huge amounts of energy. Nuclear chain-reactions occur most commonly with Uranium-235. The splitting of Uranium-235 is jumpstarted when a neutron is slammed into its positively charged nucleus. This results in two lighter isotopes (Barium-141 and Krypton-92), whose total mass is less than that of the original Uranium isotope. The "mass defect" is converted into various forms of energy. A neutron is also released, triggering the afore-mentioned chain reaction as each additional neutron causes further collisions with existing Uranium isotopes in the environment.

This reaction can also occur "spontaneously" if the original nucleus is highly unstable. The resulting products are radioactive isotopes, but do not release enough energy to generate usable power or make a bomb due to the lack of a chain reaction.

Introductory video clip on nuclear fission.
In a nuclear bomb, this must happen very quickly to make a very big explosion. The amount of energy released in the explosion is measured in kilotons. One kiloton is the same as the energy of one thousand tons of TNT (trinitrotoluene).

In a nuclear reactor this must happen slowly to make heat. The heat is used to boil water into steam, which turns a steam turbine to generate electricity.

Other websites
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 Media related to Nuclear fission at Wikimedia Commons

* Annotated bibliography for nuclear fission from the Alsos Digital Library Archived 2005-12-10 at the Wayback Machine

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