The term "chain reaction" was initially used only for designation of a number of consistently happening chemical reactions, however later the value of the term was essential is expanded. For example, now it is possible to call chain reaction result of influence of any acts or thoughts of one person on the others.
So, initially chain reaction was the chemical phenomenon. By it called such process at which emergence of an active molecule, atom or the free radical causes the whole chain of transformations of other kernels or molecules. At the same time the active particle participates in each link of a chain, i.e. in each stage of chemical reaction. It is possible to give the reaction happening between hydrogen and chlorine under the influence of light as the simplest example of unbranched chemical reaction. At the very beginning of a chain the chlorine molecule as a result of absorption of light breaks up to two atoms, each of which reacts with hydrogen. As a result very long chain of continuous chemical reactions turns out.
However there are also so-called branched chemical reactions during which two and more "chains" appear. From one active particle it turns out a little, besides each of new particles interacts in own way. Hydrogen oxidation can be an example of branched chain reaction. In interaction of oxygen and hydrogen there are two active centers at once: IT and Oh, each of which enters further reaction with hydrogen.
Besides chemical chain reactions there are also nuclear. As the active center in them serve neutrons. Nuclear chain reaction is a division of atomic kernels as a result of firing by neutrons. This phenomenon is the cornerstone of action of an atomic bomb and operation of the nuclear reactor. When firing a heavy element there is a division of its kernel into several new kernels and emergence of free neutrons. Free neutrons split new kernels, and even more neutrons and kernels appear. The chain of transformations occurs incredibly quickly, and the huge amount of energy is as a result distinguished. This process can be controlled and used as for the good – for development of energy necessary for mankind, and to the detriment – for destruction of millions of people.