how were the first atomic elements isolated and how did we know they could not be broken down further?
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By experimentation. People have always been trying to turn things into other things and coming across some things that they couldn’t break apart. Those things we’re determined to be “elements” until someone else found a way to break them.
The first good definition and method for determining something to be an element came from Antione-Laurent de Lavoisier, a French nobleman at the time of the Revolution. He formalized the theory of “conservation of mass” which states that matter (atoms) are not created or destroyed in chemical processes, they just recombine in different ways.
This theory meant there was now a method for evaluating if something was an element. You would weigh all the substances before and after a reaction. If a substance weighed less afterwards, it meant that it could not be an element. Some part of it had been broken away therefor it had been composed of multiple parts.
Lavoisier followed this method and tested everything he could get his hands on. He ended up producing the first modern list of elements, 33 in total including light and caloric, and discovered oxygen and several other gasses.
An example experiment to discover oxygen: When limestone (CaCO3) is heated in a furnace it produces quicklime (CaO). It was already known that this reaction also produced a gas, if you heat limestone in a sealed container it will inflate an attached bladder/balloon. This gas was called “fixed air” because it was air that was somehow originally attached to the limestone rock.
Lavoisier took this fixed air (CO2) and tested on a bunch of stuff. Some of the stuff he tested it one was burning phosphorus and burning wood. He found that these things would extinguish in the presence of “fixed air” and most importantly that the resultant substances would weigh exactly as much as the original substance PLUS the weight of the fixed air. He kept experimenting by adding fixed air to metals (calcination) to see if it combined and heating them to see if they produced their own types of air.
Others had done this sort of thing before, but what set Lavoisier apart was that he quantified and measured everything before and after which allowed him to track the movement of atoms across multiple reactions.
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