These atoms are made up of protons (positively charged) and neutrons within a nucleus, surrounded by electrons (negatively charged). The number of protons an atom has in a nucleus (the atomic number on the periodic table) determines how many electrons there will be surrounding, or orbiting the atom.

The electrons are used in chemical reactions and are what bind atoms together to make molecules.

The electrons orbit an atom in what are known as 'shells'. The first shell can contain 2 electrons. When this is full the electrons start to fill a second shell. This second shell becomes full at 8 electrons and so any further electrons begin to fill a third shell and so on and so forth.

The number of electrons in the outer shell determine the chemical behaviour of an atom.

Atoms that have full outer shells are very stable and inert. All atoms are trying to reach this stable state. To do this atoms with unpaired or single electrons on the outer shell (like the oxygen atom in the picture) will react with other nearby atoms in 1 of 2 ways:-

1. By stealing or donating electrons to empty or fill its outer shell.
2. Bonding with other atoms to share their outer shell electrons forming molecules.

Normally the bonds that bind molecules together do not split in a way that would leave an unpaired electron.

However when it does happen the molecule becomes a free radical. This free radical molecule will immediately look for another molecule to steal an outer shell electron from. When it does this the robbed molecule then becomes the free radical, and the process continues.