In the early 1800’s English chemist, physicist, and meteorologist John Dalton proposed his atomic theory.
Dalton developed his atomic theory by expanding upon the works of Lavoisier (Law of Conservation of Mass) and Proust (Law of Constant Composition). He was studying the composition of compounds and the ratios between the elements that compose them.
For example, carbon and oxygen can make two compounds:
carbon monoxide(CO) and carbon dioxide(CO2)
If we have a fixed amount of carbon say 100g it may react with either 133g of oxygen or 266g of oxygen to form either compound, producing a ratio of 1:2 for the mass’s of oxygen that will react with this specific mass of carbon. In other words these atoms are combining in specific proportions.
In an attempt to explain how and why elements would combine with one another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated his atomic theory.
Think about it like this, if we have a specific mass of carbon interacting with a specific mass of oxygen how can they combine in specific ratios unless they have individual particles that themselves can combine to form ratios of 1 carbon (atom?) for every 1 or 2 oxygen (atoms?).
This consideration led Dalton to propose his atomic theory.
1 Elements are made of extremely small particles called atoms. 2 Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. 3 Atoms cannot be subdivided, created, or destroyed. 4 Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5 In chemical reactions, atoms are combined, separated, or rearranged. There was also a sixth theory proposed by Dalton, the “rule of greatest simplicity” – if two elements form only one compound, assume the compound atom has only one atom of each element. This theory has since been disproven, and Dalton’s Theory expanded upon. At this point in history we know that there are these things called atoms that canot be subdivided created or destroyed and are specific for every element. In addition, we know that these atoms interact in particular ways to produce chemical compounds but what of the nature of the atom, and its subatomic particles? Sources :