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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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.
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