system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did people figure out what was and what was not an element?

In the 21st century we know about atoms, neutrons, quarks and all that and we can describe what an element is through them. How did we make lists of elements before we knew about those?
gpt
for most of history there was a lot of guesswork to it. in fact, if you look back at the historical names of some elements, you can see they would sometimes mistake a compound of 1 thing for an element of another. a lot of the highly reactive elements went unrecognized for a long time.

some of the biggest contributors to chemistry are the mol, ideal gas law, and colloidal properties. to state simply, these things care about the number of molecules involved. once you know how many molecules you're dealing with, you can figure out a lot of properties of those molecules. by comparing them with known elements, you can get a good idea of whether or not you have an element yourself.