A highly reactive molecule or atom with an unpaired electron. The species is often represented by a formula with a single dot as the unpaired electron (see immediately below). Important atmospheric examples include atomic chlorine, chlorine monoxide, NO and NO2. [Organic Chemistry; Wade, L. G.; Prentice-Hall, New Jersey; page 125; 1991.] [Environmental Chemistry; Manahan, Stanley E.; Lewis Publishers; Boca Raton; page 218; 1994.]