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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We experiment. For example, water H2O can be broken apart into H and O. SO we can see that if you do something like running electricity through water it becomes two different kinds of gasses, ergo water is a compound molecule.
Other experiments might include heating it up or exposing it to other kinds of chemicals and observing their reactions.
Announcing the discovery of a new element was sometimes contentious in the days before chemical behavior was really well understood.
Once you believed you had isolated a pure element there were a few things you could do to try and verify it.
Heat it. Cool it. Freeze it. Melt it. Electrify it. Vaporize it.
Hit it with the harshest conditions you can muster in the lab. An element should resist them all and return to the original state – it can’t possibly decompose any further.
Compound molecules will eventually give up under harsh enough conditions and vaporize or electrolyze into their constituent elements.
Water fragments into hydrogen gas and oxygen gas when you electrocute the hell out of it, and then you’ll observe that the gas you collect at one electrode keeps a flame lit and the gas at the other electrode doesn’t – suggesting two separate products.
Once we had discovered a good chunk of the elements it started to become clear that there were trends in how they behaved based on their atomic mass, and so Dimitri Mendeleev effectively predicted the remaining undiscovered ones and some of their properties in 1869.
After that we knew *exactly* what to look for when filling the gaps.
To be frank, scientists do not use electron microscopes to identify compounds.
There are several ways to determine the types of atoms in a molecule. A flame test is probably one of the oldest. One can determine nitrogen (Keldjahl analysis) or carbon and hydrogen and many other elements using destructive techniques like acid or flame digestion followed by qualitative analysis (there is a flow chart that we used to learn). Um…early scientists used to taste the compounds as well. I cannot endorse that.
Here is a small section on [qualitative analysis](https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Qualitative_Analysis/Confirmatory_Tests). Libretexts has lots more.
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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