I'm reading a book about the elements, and it says all the time: discovered/isolated by X in a year.
But how did the people know about the elements before the quantum theory? How did they know they were in front of a new "element"?
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$\begingroup$ Lots of experimental work, and they got it wrong many times as well. See here the "elements" described by Lavoisier in his Elementary Treatise on Chemistry - some of them are not really elements. $\endgroup$– Rafael LCommented Sep 5, 2022 at 18:09
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$\begingroup$ Consider for history related chemical questions hsm.stackexchange.com $\endgroup$– PoutnikCommented Sep 6, 2022 at 5:28
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How did we know about the existence of elements before quantum theory?
The short answer is that the old quantum theory had nothing to do with element discovery. Elements were considered elements after a thorough classical chemical analysis which included lots of gravimetry (weighing of precipitates), and observation of their novel chemical or physical properties. Phosphorus, for instance, was discovered by a German alchemist who collected buckets and buckets of urine in search of a recipe to make gold. One fine day, upon strongly heating dried urine residues strong with sand/coal he discovered a waxy substance which glowed in the dark for hours. Quantum theory had no part to play!
Modern textbooks teach science as a success story. In fact, science is also a story of many failures that nobody wants to record. Many elements are pre-historic like carbon, iron, copper.
When Bunsen and Kirchoff developed the spectroscope, it allowed them to see the spectrum of chemical compounds in a flame or in gas discharges. These new colors and patterns in the spectrum revealed brand-new elements. Cesium, rubidium, thallium and many gases were found this way. Helium was discovered in the Sun first! As before, no old quantum theory was involved, since the initial focus was on observation rather than determining the reason for things to happen. Even the electron and the proton were not needed to discover those elements. It was pure chemical analysis of newly discovered rocks and materials.
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1$\begingroup$ They also discovered coronium. Only later did the true identity of this element -- iron heated to millions of degrees and thus highly ionized -- come to light, and with it the real discovery of the high temperature in the Sun's corona. $\endgroup$ Commented Sep 6, 2022 at 1:29
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1$\begingroup$ Oscar, there is a very nice paper on forgotten names of the elements: link.springer.com/article/10.1007/s10699-013-9326-y $\endgroup$– ACRCommented Sep 6, 2022 at 1:31
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$\begingroup$ But how did they know it was an "element", I mean we know an element it's an atom with unique number of protons now, but what was an "element" for them before that? how did they know it's an element and not a compound? I think it's answered here: chemistry.stackexchange.com/questions/137140/… $\endgroup$– EnriqueCommented Sep 16, 2022 at 11:25