However, rather than seek out something to react with, there is an easier way for an oxygen atom to achieve stability. If we took two Oxygen atoms, each with 6 electrons in their outer shell, there are a number of options :

1) One oxygen atom could give 2 electrons to the other oxygen atom, producing one stable atom with 8 electrons in its outer shell (ideal), but one unstable atom with an outer shell of 4 (not ideal) - so that is not the answer.

2) The two oxygen atoms pair up and share electrons with each other.

In the diagram above, we have coloured the electrons differently for our explanation, but the two oxygen atoms are identical.

By pairing up and sharing, each oxygen atom effectively has the ideal 8 electrons in its outer shell. The atom on the right has 6 orange electrons of its own, and two blue electrons shared with the atom on the right. The atom on the left has 6 blue electrons of its own, and two orange electrons shared with the right atom. As long as they stay together, the atoms are stable.

This is called COVALENT bonding, which means that electrons are shared between 2 atoms. Oxygen atoms are so reactive, that the element does not exist naturally as atoms. Oxygen atoms always pair up and exist as units of two atoms. This is an oxygen MOLECULE (as shown above) which you will see written as O2 (pronounced "oh two").

Many other gases also exist as molecules, not atoms. Hydrogen has only one electron in its outer shell, and gets the ideal two electrons by existing as H2 and sharing its electron with another hydrogen atom. Nitrogen, with 5 electrons in its outer shell, needs to get another three, so it shares 3 electrons with another nitrogen atom to get its ideal outer shell of 8.

When these molecular gases get involved in reactions, the bond in the molecule must first be broken apart so that the individual atoms can react. The more electrons that are shared in the bond, the stronger and more difficult to break the bond is. Molecular gases with stonger bonds are less reactive than those with weaker molecular bonds.

So we know that the bond in hydrogen gas (H2 - with one shared electron) will be easy to break, so hydrogen will be very reactive. Oxygen gas (O2 - a stronger bond with two shared electrons) will be quite reactive, and Nitrogen (N2 a strong bond with three shared electrons) will be faily unreactive.

Although you can split O2, H2, and N2 molecules into two atoms, they are still a elements. N2 is pure nitrogen. However when two different atoms react together and form a bond, the result is called a compound. The most common compound, using atoms that we are already familiar with, is water (H2O).

In a water molecule, the two hydrogen atoms each share one electron with the oxygen. This gives each hydrogen atom two electrons in their outer shell (the ideal number for shell 1), and the oxygen atom eight in its outer shell (the ideal number for shell 2). Each atom has an ideal outer shell, so we would expect water to be a stable and unreactive compound (and indeed it is).