This is really the fundamental assumption of chemistry, and thus the measurement of mass is the fundamental process underlying much of chemical work. Another fundamental concept is that of elements. Once conservation of mass was accepted it was noted that combinations of various elements to form compounds nearly always result in the formation of compounds with constant proportions by mass. But of course there is more than one way to combine many elements to give compounds. These various laws imply that matter is made up of small discreet units, which we call atoms .
All matter is composed of ultimately small particles, called atoms.
Atoms are permanent and indivisible - they can neither be created nor destroyed. Elements are characterized by their atoms. All atoms of a given element are identical in all respects. Atoms of different elements have different properties. Chemical change consists of a combination, separation, or rearrangement of atoms.