radical

In chemistry, an atom or group of atoms carrying an unpaired electron, which is therefore highly reactive and often impossible to purify or keep for any length of time.

A radical can either recombine with another radical (creating an electron pair and thus a stable compound) or attack a stable molecule and thereby create a new radical and start a chain reaction. However, organic chemists can create reasonably stable radicals by surrounding the atom carrying the unpaired electron with bulky molecular groups that block or slow down its reaction.

Ozone is a radical, as are some of the chlorine compounds that are liberated from organic pollutants and start the chain reactions that ultimately damage the ozone shield.