Another common meaning for the word which Webster does not cover (due to the fact that nuclear physics wasn't as big a field in 1913 as it is now) is the process where an atomic nucleus splits into two or more lighter nuclei, releasing radiation, fast neutrons and large amounts of energy in the process. Today, the word 'fission' generally refers to this meaning in popular conversation, due to the importance of nuclear energy to all of our lives (although nuclear fission would be a better term - as Webster notes, the unadorned term 'fission' has other meanings).

Unstable radioisotope nuclei will fission spontaneously, and certain other nuclei not normally considered 'unstable' (such as 235U) may be made to fission with the application of slow neutrons. These slow neutrons are usually created in a chain reaction, since they are otherwise quite expensive to generate. The neutrons have to be slow because 'fast' ones aren't as readily captured by atomic nuclei, and are thus 'wasted'. Since most neutrons resulting from fission are 'fast', they must be slowed down by a moderator such as heavy water or graphite to cause further fissionings.

Side note: Only nuclei heavier than iron nuclei will yield energy when fissioned, and only nuclei much larger than iron nuclei will yield economic amounts of energy when they do so. Fissioning an element that is lighter than iron actually consumes energy. The converse is also true - fusing elements lighter than iron yields energy, fusing elements heavier than iron consumes it. Iron is therefore the ash of nuclear fires of all kinds. Supernovae are caused when a massive star runs out of lighter elements to fuse than iron - since it cannot sustain a fusion reaction anymore, it is caused to explode. Due to the large amounts of energy present in the explosion, some iron and other nuclei are made to fuse, creating heavier elements and 'charging' the nuclear 'batteries' of heavy elements. We are all made of stardust!
In chemistry, fission is the process by which a molecule splits into two constituent parts. This occurs when one of the chemical bonds between atoms in the molecule is broken.

In homolytic fission, the two electrons from the broken bond are shared between the resulting species. This means that each species contains an unpaired electron in an outer shell. They are therefore highly reactive, and are known as free radicals. Homolytic fission occurs when the two atoms being separated have a similar or identical electronegativity - that is, they have roughly the same ability to attract electrons to themselves.

One example is the splitting of halogen molecules (X2) by ultraviolet radiation. Chlorine, Cl2, splits into two chlorine atoms, 2Cl•. These atoms are free radicals and their reactivity may be used to initiate useful chain reactions.

In heterolytic fission, the two electrons from the broken bond go to the same species. This occurs when one species is significantly more electronegative than the other. Heterolytic fission results in a negatively charged anion, which received both electrons, and a positively charged cation, which received neither.

For example, water, H2O, may split heterolytically into a hydroxyl ion, OH-, and a hydrogen ion (a proton), H+.

Fis"sion (?), n. [L. fissio. See Fissure.]

1.

A cleaving, splitting, or breaking up into parts.

2. Biol.

A method of asexual reproduction among the lowest (unicellular) organisms by means of a process of self-division, consisting of gradual division or cleavage of the into two parts, each of which then becomes a separate and independent organisms; as when a cell in an animal or plant, or its germ, undergoes a spontaneous division, and the parts again subdivide. See Segmentation, and Cell division, under Division.

3. Zool.

A process by which certain coral polyps, echinoderms, annelids, etc., spontaneously subdivide, each individual thus forming two or more new ones. See Strobilation.

 

© Webster 1913.

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