What are Free Radicals?

Asked by: Mike Neville


Atoms are bonded together when they share or transfer electrons to form molecules. A covalent bond is formed when a pair of electrons is shared. When the bond breaks, it can occur in one of two ways:

The more common way is 'heterolytic cleavage' where one of the atoms retains both of the bonding electrons, and the other takes none. This generally results in the forming of ionic species.

e.g. H-H --> H+ + H-
In this case, the H- has taken the electron pair.

However, it is possible that both atoms retain one electron each in a process called 'homolytic cleavage'. The two atoms/molecules that are formed each contain an unpaired electron, making it highly unstable and reactive. These are called 'Free Radicals.

e.g. H-H --> H· + H·
The · next to each H shows they are free radicals (each has an unpaired electron). These highly reactive molecules will then react quickly with another nearby molecule.

Due to how reactive they are, free radicals can be very dangerous. Free radicals occuring in the human body can attack and damage cells. This is one reason why fruit and vegetables are so good for us is that they contain anti-oxidants which react with free radicals and stop them harming us.

However, free radicals are also very useful in some reactions. For example one method of polymerisation, the process by which plastics are made, depends on free radicals reacting with other molecules, producing more free radicals and making long chain polymers.

To summarise, free radicals are highly reactive short lived molecules that have one or more unpaired electrons.
Answered by: Simon Hooks, Physics A-Level Student, Gosport, UK