Editing History of atomic theory (section)

==Groundwork==
Working in the late 17th century, [[Robert Boyle]] developed the concept of a chemical element as substance different from a compound.<ref name="Whittaker"/>{{rp|293}}
Near the end of the 18th century, a number of important developments in chemistry emerged without referring to the notion of an atomic theory. The first was [[Antoine Lavoisier]] who showed that compounds consist of elements in constant proportion, redefining an element as a substance which scientists could not decompose into simpler substances by experimentation. This brought an end to the ancient idea of the elements of matter being fire, earth, air, and water, which had no experimental support. Lavoisier showed that water can be decomposed into [[hydrogen]] and [[oxygen]], which in turn he could not decompose into anything simpler, thereby proving these are elements.<ref>Pullman (1998). ''The Atom in the History of Human Thought''. p. 197</ref> Lavoisier also defined the [[law of conservation of mass]], which states that in a chemical reaction, matter does not appear nor disappear into thin air; the total mass remains the same even if the substances involved were transformed.<ref name="Whittaker"/>{{rp|293}} Finally, there was the [[law of definite proportions]], established by the French chemist [[Joseph Proust]] in 1797, which states that if a compound is broken down into its constituent chemical elements, then the masses of those constituents will always have the same proportions by weight, regardless of the quantity or source of the original compound. This definition distinguished compounds from mixtures.<ref>{{Cite web|title=Law of definite proportions {{!}} chemistry|url=https://www.britannica.com/science/law-of-definite-proportions|access-date=2020-09-03|website=Encyclopedia Britannica|language=en}}</ref>