This was noticed early by people, and Greek thinkers, about 400 BC, used the words ‘element’, and ‘atom’ to describe the differences and smallest parts of matter. These ideas survived for 2000 years while Aristotle’s concepts, such as ‘elements’ of Earth, Fire, Air, and Water to explain ‘world stuff’, came and went
Lavoisier It was Lavoisier who wrote the first extensive list of elements - containing 33 elements. He distinguished between metals and non-metals, dividing the few elements known in the 1700's into four classes.
John Dalton Then John Dalton made atoms even more convincing, suggesting that the mass of an atom was it's most important property.