In this section, we learned how chemical reactions take place. At the start, there is the original molecule/reactant that when acted upon, the outcome is a product of a new arrangement of the molecule. Always when a chemical reaction there is a bond being broken, being created, or both are taking place.
When there is a change in chemical structure, there is never a loss or gain of any atoms. So if there is H2+O2 you cannot make a single water molecule, due to the leftover atom. But if there were to be two more hydrogen atoms, there could be two water molecules. This is called the Law of Conservation of Mass. Also, in order to balance a chemical equation, you must change the coefficient in front of the number, so that is matches and a proper reactant/product is created.