Between 1800 and 1805 Dalton began publishing the atomic weights of elements.  The first six elements included hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus.   He reasoned (incorrectly) that one hydrogen atom combined with 1 oxygen atom to form water.  The weights he assumed were hydrogen (weighed “1” unit) and oxygen (weighted “8”) units.  Dalton did not realize that combinations could include two hydrogen atoms and one oxygen atom.  However, the ratio he assumed of 8:1 (mass) was amazingly accurate! Dalton started the world of chemistry to think about how to determine the weight (now referred to as mass) of elements. 

Amedeo Avogadro (1776-1856) in 1811 developed the concept of a molecule while experimenting with how oxygen combined with hydrogen to form water.  Although trained in law, he turned to mathematics and physics later in his career.  He mathematically calculated that the same volume of any gas would have the same number of molecules at the same temperature and pressure.  Italian chemist Stanislao Cannizzaro (1826-1910) refined this new concept of a molecule, which helped explain the difference between an element and when elements combine.  The concepts of atoms and molecules started to take form.