Free Radicals

Occassionally molecules appear that contain an odd number of valence electrons. Obviously,  with a uneven number of electrons these molecules can't possibly have all of their electrons paired.  Most of the molecules with uneven numbers of electrons have a central atoms from odd numbered groups of elements.  For example, nitrogen  from Group 15 and any halogen such as chlorine from Group 17.  These species are called free radicals .  They contain a lone (unpaired) electron, which makes them extremely reactive.  Free radicals react to pair up their lone electrons.  For example, when two NO₂ molecules collide the form dinitrogen tertraoxide N₂O₄, so that each N attains an octet.  Free radicals are usually shown by the addition of a dot preceeding the molecular formula of the free radical.  For example OH to signal the presence of an unpaired electron.

                     
We often hear about free radicals and their role in the development of cancer and other diseases.  However, most general chemistry courses never touch on the question of free radical and their chemistry.  In the atomospheric sciences, free radical behavior  is a key component in the processes that lead to photochemical smog.  Near the Earth's surface, where gas densities are substantially  greater than in the stratosphere, such free radicals react with other substances almost immediately and as a result, they are not important players in naturally occuring chemical reactions. But in the rarified outer atmosphere, free radicals like individual O , N, and H atoms have significant lifetimes because gas molecules are very far apart and collisions occur infrequently. Inside biological systems free radicals can be deadly.  They abstract, or pull off,  H-atoms from cell components to form electron-pair bonds and new free radicals.  Consequently, the number of free radicals increases dramatically while  genes and cell membranes are ruptured in the process.

 Most of the free radicals that are important players in atmospheric chemistry have their unpaired electron located on a C, O, H or halogen atoms.  The specific atomic location can be shown by placing a dot above the symbol of the atom that is missing one electron.   The unpaired electron exists as a nonbonding electron localized on one atom rather than as a bonding electron shared between two atoms.  Usually, an atom with one unpaired electron forms one fewer bonds than you might expect since its unpaired electron is not in acutual use as a bonding electron.  A carbon atom that would normally form 4 bonds would only form 3 bonds in a free radical if it were the site of the radical.  A halogen or hydrogen free radical forms no bonds instead of the 1 bond you would expect if it is the radical site.

In the early 1960s scientists realized that free radicals play a substantial role in the catalytic destruction of stratospheric ozone.  Free radicals can be expected to have a longer life in portions of the atmosphere where gas molecules are further apart since there will be fewer collisions. Free radicals react efficiently with ozone by abstracting (removing) an oxygen atom from ozone.