Free radicals

In our body, the majority of the electrons exist in a pair state. Paired electrons are quite stable.

Under some circumstances (pollution, radiation...) the bond between these two electrons is broken.
They split up and the species bearing them become highly unstable and reactive.

When they split up we have two situations:

1- They stay together and form a molecular fragment called ions. (The atom that got the two electrons is negatively charged and the one without the electrons is positively charged).

Example: Sodium chloride salt splits up into a chloride anion Cl- and a sodium cation Na+

2- Each electron goes its different way and each atom gets an electron. These atoms have an unpaired electron and are called free radicals.