So, initially the chain reaction was a chemical phenomenon. She called this process in which the appearance of an active molecule, atom or free radical causes a chain reactions of other nuclei or molecules. The active particle is involved in every link of the chain, i.e., in each stage of the chemical reaction. As a simple example, unbranched chemical reaction can cause a reaction occurring between hydrogen and chlorine when exposed to light. At the beginning of the chain a molecule of chlorine as a result of absorption of light splits into two atoms, each of which reacts with hydrogen. The result is a very long chain of continuous chemical reactions.

However, there are so-called branched chemical reaction in which there are two or more "circuits". From one active particle is somewhat, though each of the new particle interacts in its own way. An example of a branched chain reaction may be the oxidation of hydrogen. The interaction of oxygen and hydrogen arises from two active site: - IT, each of whom shall enter into further reaction with hydrogen.

In addition to chemical chain reactions, there are also nuclear. Active center, they are neutrons. Nuclear chain reaction is the fission of atomic nuclei as a result of bombardment by neutrons. This phenomenon is the basis of the atomic bomb and nuclear reactor. During the shelling of heavy elements is a division of its nucleus into several new nuclei and free neutrons. Free neutrons split the new kernel, and you have more neutrons and nuclei. A chain of changes happening really fast, and as a result releases massive amounts of energy. This process can be controlled and used for good – to establish the necessary mankind of energy, and harm – for the destruction of millions of people.