Reaction Stoichiometry
According to atomic theory, in a chemical reaction, atoms are not created nor destroyed. What does occur is that the connections (bonds) between atoms change. As an example of this, consider the reaction of hydrogen gas (H2) and oxygen gas ( O2) to form water (H2O).
In the diagram below, the hydrogen atoms are represented by blue spheres and the oxygen atoms are represented by red spheres. In the reactants (starting material), the hydrogen atoms are connected in pairs to form H2 molecules and the oxygen atoms are connected pairwise to form O2 molecules. In this example, there are a total of twelve (2x6=12) hydrogen atoms and six (2x3=6) oxygen atoms. The result product must contain these same eighteen atoms.
| Reactants | Products | ||
 |
----> |  |
|
| # H atoms | 12 | 12 | |
| # O atoms | 6 | 6 |
In the example below, an equal number of hydrogen and oxygen atoms are present initially. However, since the product contains twice as much hydrogen as oxygen, some of the oxygen is left over and doesn't react.
| Reactants | Products | ||
 |
----> |  |
|
| # H atoms | 12 | 12 | |
| # O atoms | 12 | 12 | |
| # O in O2 | 12 | 6 | |
| # O in H2O | 0 | 6 |