Last revised by Patrick J Rock on 13 May 2021

Radicals (formerly called free radicals) are uncharged atoms or molecules in which an electron orbit has a single unpaired electron.


Historically the International Union of Pure and Applied Chemistry (IUPAC) also used the term radical for any joined up group of atoms forming a side-group of a larger molecular entity, e.g. carboxyl radical. By contrast, unpaired electron species were called free radicals. However the first use of radical has been abandoned such that IUPAC uses the term group instead, therefore carboxyl group not radical. So it has been decided that the descriptor 'free' is now superfluous as in general radicals are free 1.

When radical species are written down, e.g. "∙H", a dot to the left of the atom is used to denote the presence of an unpaired electron.

Also please note that the spelling is 'radical' and not 'radicle'. The latter is occasionally used in anatomy to refer to a small branch, e.g. biliary radicle.

Radical formation

Ionizing radiation produces radicals by the processes of ionization and excitation. The simplest radical is atomic hydrogen, with a single orbital electron.

Radicals are chemically reactive, achieving stability by pairing their unpaired electron with the orbital electrons of neighboring molecules. They are the primary cause of biologic damage from low linear energy transfer (LET) radiation.

They can have effects on distant molecules:

  • smaller radicals can diffuse through the medium they are in before reacting (within 1 micrometer from their formation point)
  • chain reactions can occur: the radical either transfers an electron to, or accepts an electron from, a neighboring molecule, therefore rendering the neighboring molecule itself a radical: this process may be repeated through a chain of molecules

In cells, water is the most abundant source of radiation-produced radicals, and excitation and ionization results in formation of hydrogen (∙H) and hydroxyl (∙OH) radicals

  • H2O -> ∙H + ∙OH
  • these radicals may interact with other cell molecules causing discrete changes in part of that molecule

The damaging effect of radicals is enhanced by the presence of molecular oxygen:

  • oxygen combines with the hydrogen radical to form the highly reactive hydroperoxyl radical (∙HO2)
  • thus, cells with higher O2 content are generally more sensitive to radiation

Radicals are the primary cause of biologic damage from low linear energy transfer (LET) radiation and this may occur:

  • by inactivating cellular mechanisms directly (e.g. via damage to molecules involved in cellular metabolism), or
  • by damage to genetic material (DNA/RNA)

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