Every chemical reaction starts with one or more reactants. Atoms, molecules or  free electrons can participate. They combine, swap atoms or decompose into the  reaction’s products. If a reactant is a single atom, it’s usually in ionized  form—having too many or too few electrons. Molecules may be neutral or  ionized. The reactants must be in physical contact in order to react. This  usually means they occupy the same container and are being heated or mixed  together.