A free radical is an atom or a group of atoms with an odd number of electrons. The odd, unpaired electron in a free radical seeks to pair (form a bond) with another electron. A free radical readily reacts with another atom or group of atoms. Most free radicals are so reactive they exist only for a fleeting moment.

Organic molecules contain carbon, which very readily forms four electron pairs (bonds) when reacting with other atoms or molecules. An organic free radical is a free radical form of carbon with three bonds and a single, unpaired electron.

A free radical can react with another free radical, but more often it reacts with a stable, evenly paired molecule. It does so by (1) donating an electron to the molecule, (2) removing an electron from the molecule, (3) removing a group of atoms from the molecule, or (4) adding itself to the molecule. When a free radical reacts with a stable molecule in these ways, it turns the stable molecule into a free radical. This can set in motion a chain reaction of free radical formation.