How do molecules exist in a chemical reaction?

Molecules exist in the following two forms
(i) Molecules of elements These are formed by the combination of two or more atoms of the same element, e.g. ${{O}{2}}$, H2, P4 etc.
(ii) Molecules of compounds These are formed by the combination of atoms of different elements, e.g. NH3, $C{{O}
{2}}$ etc.