Abstract
A molecule that has an unpaired electron is called a free radical and is a paramagnetic species. For example, the methyl radical is an extremely unstable and reactive species because its octet rule on the carbon is not filled. It is apparent that free radicals are unique and rare species, and are present only under special and limited conditions. However, some of the free radicals are familiar to people in their daily lives. Molecular oxygen is a typical free radical and a biradical species. Standard and stable molecular oxygen is in the triplet state (3O2), and the two unpaired electrons have the same spin orientation in two orbitals (parallel), respectively, having the same orbital energy, based on Hund's rule. Nitrogen monoxide and nitrogen dioxide are also stable, free radical species. The reactive species involved in immunity are oxygen free radicals, such as superoxide anion radical (O2–) and singlet molecular oxygen (1O2).
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