nuclear fission

splitting of an atom's nucleus into two equal parts

Nuclear fission is the splitting of the nucleus of an atom into parts (lighter nuclei) often producing photons (in the form of gamma rays), free neutrons and other subatomic particles as by-products. Fission of heavy elements is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). Fission is a form of elemental transmutation because the resulting fragments are not the same element as the original atom.
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Noun
1. a nuclear reaction in which a massive nucleus splits into smaller nuclei with the simultaneous release of energy
(synonym) fission
(hypernym) nuclear reaction

Nucleus is split into (most frequently) two fission fragments. This is the predominant energy production nuclear reaction in all trans-uranic elements since it gives out energy. The energy (E) is the mass defect, or loss of mass, m, (the fragments being of less mass than the original nucleus), times the velocity of light squared: E = mc2. See Special Theory of Relativity .

Spontaneous fission was first discovered by the Russian physicists Flerov and Petrazak in 1940 in uranium. It occurs in nuclides with too many protons, is the simplest form of dynamical instability in nuclei and can be explained in terms of the liquid drop model, as illustrated. A heavy nucleus is in a permanent state of oscillation with a frequency of the order of 10 vibrations/sec. In the course of any one vibration the incompressible nucleus is distorted to a greater or lesser extent, there being a small, but finite, probability that the distortion may become so extreme that a constriction appears between the ends of the elongated nucleus. The mutual electrostatic repulsion of the charges on the two may then be sufficient to overcome the attractive nuclear 'surface tension' forces and to cause the nucleus to rupture. The half-life to such binary fission of uranium-238 is 8 x 1015 years, while that for the competing process of alpha particle emission is 4.5 x 109 years. In any gramme of natural uranium metal there are, on average, 25 spontaneous fissions/hour. The more the number (Z) of mutually repulsive protons the greater the chance of spontaneous fission and so for Z=101 mendelevium the half life is 3.5 hours. This is the basic reason why there is an upper limit to the number of elements in the universe.
Induced fission is when the process is speeded up by hitting the heavy nucleus with a projectile such as a neutron. This fact is utilised in atomic bombs and nuclear reactors here on earth.

The division of the nuclei of isotopes of certain heavy elements, such as uranium and plutonium, effected by bombardment with neutrons. Nuclear fission causes the release of energy, additional neutrons, and an enormous quantity of heat. Nuclear fission is used in nuclear power plants and nuclear weapons. A byproduct of nuclear fission is toxic radioactive waste. See also nuclear fusion.