This is something that never actually got explained to me in chemstry lessons in school and I’ve always been curious about since.
How, before electron microscopes capable of seeing individual atoms, were scientists able to say for certain that some substances were only made of one type of atom (e.g. oxygen in oxygen gas/ozone, carbon in graphite/diamond, or iron) but others were made of multiple types (e.g. H2O)
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Because first of all, we don’t tell that something is made out of a particular element with electron microscopy.
People long ago realised that you had certain elementary substances with specific properties (mass, valency etc), but they wouldn’t be able to tell you what they actually were. That only came with the discovery of the electron and the Rutheford gold foil experiment at the turn of the 20th century… Or rather even later, with the proton and neutron in the 1920s.
If you know any chemistry, you know it has rules. Mass is always preserved, but sometimes as gas, substances combine only in certain ratios, all of those.
Well, through centuries of experiments people realised that some substances couldn’t be further refined into more elementary forms, only combined with other things. Hence elements. I believe sulphur was one of the first, but if someone knows better, please feel free to correct.
So what you had between then and the atomic theory was a lot of people combining and splitting a lot of chemicals and meticulously checking their weights to the absolute limits of their scales. And creating rules according to which chemistry seems to have worked. Once you had those elements they could only combine in certain ratios. Boom, valency. Also boom, atomic weights and moles, because the weights of them also had the same ratios.
Of course chemists started doing it on purpose when they got wise to it, but it doesn’t change the fact that it was pure guesswork supported by a mind-bogglingly tedious process of just… doing chemistry over and over really precisely.
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