Atomic Masses The original idea of relative atomic masses came from John Dalton. Others before Dalton - Lavoisier for one - had used the concept of combination of simple substances into more complex compounds. However, it was Dalton who realized that if these particles had their own unique mass they could account for the Law of Conservation of Mass. Dalton believed, mistakenly, that only one atom of one kind could combine with another. He invented a series of symbols to represent each atom. Notice that only his formula for carbon monoxide would be considered correct today. He analyzed the decomposition of water, which produces eight weights of oxygen for every one weight of hydrogen, and concluded (incorrectly, since he was using the wrong formula for water) that an oxygen "simple", an atom, was eight times heavier than a hydrogen atom. Though his idea of atoms was brilliant, his mistaken concept that compounds had only single atoms of each element caused him to reject many ideas that later turned out to be much better - including that of Avogadro. Finding the mass of atoms of a certain element is important because when you experiment on them, you can mass them, and then do the experiment and see whether or not the experiment had an effect on the atom. 
Lavoisier
