Radical, in chemistry, is a group of two or more charged or neutral atoms that have at least one unpaired electron.
Molecular oxygen has two unpaired electrons and is a common radical.
Most radicals are extremely reactive and combine with other atoms or radicals to form compounds or ions. However, some radicals, called free radicals, may exist for relatively short times unbound to any other group. Radicals play a major role in certain chemical reactions of commercial significance, including the formation of polymers. They are also important in understanding atmospheric reactions caused by sunlight. Mark S. Wrighton, Ph.D., Ciba-Geigy Professor of Chemistry, Massachusetts Institute of Technology.