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
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
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