Hydrogen and oxygen exist as diatomic (2 atom) gases, where each hydrogen atom is linked to another hydrogen atom by a single covalent bond and each oxygen atom is linked to another oxygen atom by a double covalent bond. Simply put, a covalent bond is a chemical bond between atoms were the electrons are shared.
If you mix hydrogen and oxygen together, nothing happens. It takes an energy source (like a spark or a small flame) to get the reaction started. During the reaction, the hydrogen and oxygen bonds to each other are broken and new chemical bonds form between the hydrogen atoms and the oxygen atoms. Each oxygen atom ends up with 2 hydrogen atoms bound to it.
The chemical equation looks like this:
2H2 + O2 --> 2H2O
So, for every molecule of oxygen, you need 2 molecules of hydrogen to react to form 2 molecules of water, or you end up with hydrogen or oxygen left over.
(published on 06/02/2008)