how were the first atomic elements isolated and how did we know they could not be broken down further?
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I believe that the earliest work involved breaking down of compounds into the elements, and identifying that a constant mass ratio (weight ratio) was seen in the products, and the reverse, that when reacting two different substances, the mass of reactants that were lost and the mass of the product also conformed to the mass ratio rules. Part of this work involved the production of “pure” materials which would not break down further.
Working with these observations led to the tabulation of elements by mass. Playing around with density relationships (mostly in gases) led to a refinement (correction) of the raw ratios to unit ratios. That is, when water was broken apart, you got 2 mass units of hydrogen (H) (which wasn’t called H at first) and 16 mass units of oxygen (O) (“phlogiston” in early work), but that 16 mass units would only give half the pressure that hydrogen (antiphlogiston) would in the same volume. This was taken to indicate that there was half as many “atoms” (the thing they decided must be the smallest unbreakable unit, just to have a marker, a base line, to work from; did not prove it at first, they assumed it) of oxygen as hydrogen in water.
So, this starts the thinking along the lines of what we now call “stoichiometry”, which is that elements combine in integer ratios that reflect the number of each “atom” in a specific compound.
Seeing that H was the “lightest” (least dense) “element”, they assigned that a mass value of 1, and worked up the scale from that.
And then the fun really started as they started to fill in the periodic table and noticed how certain patterns in chemical behavior arose in regular ways.
I tried to keep this short and simple, so it is not exact, just a rough summary.
The idea of the unit size of atoms only really came about once the atom model was combined with charge and electricity. That is, once the charge behavior of an atom was identified (like H would act like H+), and measurements of current used to quantities of product were correlated, the unit charge value and number of charged units was figured out (different experiments produced results that made lines, rough lines, when plotted in terms of mass and current, revealing unit charge from the slope, and unit charge leads to a way to calculate number of units per quantity of electricity. The size of the atom turned out to be extremely small.
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