Antoine-Laurent Lavoisier established the law of conservation of mass, determined that combustion and respiration are caused by chemical reactions with what he named “oxygen,” and helped systematize chemical nomenclature, among many other accomplishments. through a series of experiments found that the total mass of products and reactants in a chemical reactions is always the same. This led to the theory of the law of conservation of mass.
16 mass units + 2 mass units= 18 mass units
Mendeleev figured out that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.
Dalton's atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atomsof an element were identical, different elements had atoms of differing size and mass.