Nuclear fission is the splitting of the nucleus of a large atom, usually of a heavy element, in order to produce two smaller atoms, and release a large amount of energy. A nucleus may split because of radioactive decay, a process by which energy is lost by an unstable nucleus in terms of mass and radiation, or because of collision, or bombardment, with a sub-atomic particle, such as a proton or a neutron. The energy released during nuclear fission has been harnessed to create extremely powerful bombs, and in power plants to provide a source of energy and electricity to consumers in their homes and places of business.
Initially, scientists, including Enrico Fermi, began to bombard samples of the heavy element Uranium with neutrons. This was first believed to have created the first elements heavier than Uranium, however Ida Noddack, a chemist, proposed that the bombardments could have actually split the Uranium atoms into smaller atoms. Noddack did not provided any further theoretical evidence to support this claim, but in December 1938, scientists Hahn and Strassman found the first solid evidence of nuclear fission when they discovered nuclear fission among the decay products of their experiments with Uranium (https://www.aps.org/publications/apsnews/200712/physicshistory.cfm).
Nuclear fission occurs through a long and detailed process based on many factors; including atomic mass, surface area, and forces within the atomic nucleus. The process begins with the atomic anatomy of a given isotope, which determines its stability and the possibility of it being broken into smaller atoms.
Despite its atomic mass being a certain number, the mass number of the nucleus of any given element or isotopes is actually less than the total mass of the neutrons and electrons which constitute it; this is called the “mass defect” of an atom. An atom’s mass defect is also a measure of its binding energy: the energy released when a bond between particles is formed in a nucleus, and the amount of energy required to break apart that nucleus, in turn making this a measure of the stability of any given atomic nucleus. Peaking around element 56, iron (Fe), this means that any element with a higher atomic number would be more stable if it were broken into two smaller nuclei rather than one large nucleus with a low bonding energy.
Even though there are strong attractive forced between the particles that make up an atom’s nucleus, these only act on closely neighboring protons or neutrons. Since particles on the surface of the nucleus inherently have fewer neighbors with which they can interact, the nucleus forms into a sphere, whereby the volume of the nucleus is contained with the least possible surface area. If the nucleus is highly unstable, that is, if it has a very low bonding energy, it may be in higher danger of splitting if it is excited by an outside impetus and the surface begins to deform and create a shape other than that of a sphere. This would mean that the particles on the surface have even less attraction between themselves and the particles surrounding them, which risks lowering the stability of the nucleus as a whole.
However, fission may also be induced in a process known as induced fission. The opposing forces of attraction and repulsion inside of a nucleus create a barrier in the potential energy contained within a system of an atom; the system is the configuration of the atom in its current state. If the nucleus of that atom is excited or energized to the point here its energy is greater than, or sometimes equal to, the state defined in the barrier, the atom may experience fission. This excitation may occur in the form of gamma-rays, or by disturbing the shape of the nuclear drop—the “clump” of protons and neutrons in a nucleus—to the point where it is no longer stable and its energy exceeds that defined in the barrier of the system. The addition of a particle, such as a neutron, to the nuclear drop also adds activation energy (bonding energy) to the total energy of the nucleus which can make it even more unstable than it was to begin with. All additional energy supplied to complete the reaction which results in the nucleus splitting to form two lighter atoms comes from the kinetic energy that is received when a particle collides with the nucleus and moves it.
If the nucleus of a very heavy atom experiences fission, there are many possible fragments pairs which may be produced, depending on how the protons and neutrons were distributed in the nucleus of the original atom. The fragments experience powerful repulsion forced from each other, and move away from each other at a rate determined by the size of the respective new atoms and the amount of kinetic energy they can maintain. Initially, the two fragments will move away from each other so fast that some of the outer electrons will be stripped away from the atom, making these fragments highly charged. Internal energy in the newly formed fragments also causes neutrons to be expelled from the nuclei, and gamma-ray radiation is also emitted. The emission of radiation (electromagnetic) almost coincides with the initial reaction, it happens so quickly. The kinetic energy of these atoms and particles colliding with other atoms is transferred into ionization energy and heat, even though the fragments will only move a few centimeters through the air in which the reaction takes place.