Highly reactive atoms or molecules with incomplete (electronic) octets and therefore uneven numbers of electrons. (In the case of hydrogen radical this is an incomplete duet.) Free radicals species are very electrophilic, will abstract atoms from other molecules to complete their octets, and will, in the process, generate new radicals. In the atmosphere, most free radical species have short life times; however, they can promote the conversion of oxygen to ozone and also take part in the catalytic cycle of ozone destruction (see chlorine and chlorine monoxide).
[Organic Chemistry; Wade, L. G.; Prentice-Hall, New Jersey; page 125; 1991.] [Atmospheric Change; Graedel, T. E. ; W. H. Freeman and Company; New York; page 141; 1993.]
Source: Atmospheric Chemistry Glossary