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Nuclear Fission
The basic component of nuclear fission is an atom. An atom is the tiniest particle or unit within an element. Atoms possess characteristics of the element and cannot be broken into smaller parts while still retaining the element's properties. An atom contains electrons, and also a nucleus which consists of protons and neutrons.
Nuclear fission occurs when the nucleus of an atom divides into two nuclei. When this division happens, a large quantity of energy is released and products are formed.
Products
Products created from nuclear fission are radioactive. The energy created by nuclear fission comes in the form of heat and radiation. In addition to this energy and two nuclei, two or three neutrons are produced.
Cause
Nuclear fission can occur spontaneously in nature. For example, uranium and other elements undergo spontaneous fission within the Earth's core. The energy produced from the fission generates heat and drives plate tectonics, which is the premise for geothermal energy. Also, the radiometric dating of rocks is made possible by nuclear fission that occurs naturally.
In addition to spontaneous reactions, nuclear fission occurs under controlled, man-made mechanisms that irradiate certain elements with neutrons in order to produce reactions.
Chain Reaction
The neutrons that are produced by nuclear fission are free agents. These free neutrons can, in turn, incite another atom to undergo nuclear fission. The free neutrons created from that second fission can incite another atom and so on. This process is called a chain reaction. Because elements can be composed of numerous atoms, millions of chain reactions can occur in less than a second, depending on the type and size of the element sample being used. Not all elements undergo nuclear fission. Heavy elements such as thorium, uranium and plutonium are most commonly used to generate chain reactions, which among other things, are used to create military weapons.
Discovery
The nuclear fission reaction was discovered in 1938, by accident. German physicists, Fritz Strassmann and Otto Hahn were conducting experiments that involved bombarding a somewhat random variety of elements with neutrons. While all other elements became radioactive with slightly changed properties, it wasn't until uranium was irradiated that they stumbled upon the dramatic and unexpected results of split atoms. The reaction was named "nuclear fission" because it reminded the scientists of cellular fission, which is a process that splits a cell into two parts.