Radicals are formed by the cleavage of an atom or molecule and can be grouped into three categories depending on their source. They can come from atoms, (e.g., H, F, Cl), inorganic molecules (e.g., such as OH, CN NO), or organic molecules (e.g., CH3 or C2 H5). In some areas of chemistry the term radical is used to indicate a reaction intermediate, which exists in nature for very short periods of time. However, the term is more commonly used to describe chemical species that persist long enough to react with other molecules to form more radicals in a cascading effect. This cascade effect can create sustained reactions in chemical and biological systems.