ELI5, how did people discover the mass of helium and other elements that are lighter than most of the atmosphere?

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I’ve tried searching for it, but all I get as a response is how to calculate the mass of a baloon filled with helium knowing the atomic mass of helium, which doesnt help. But I haven’t been able to know how the atomic mass of Helium was discovered/calculated and I have been searching for an explanation for a while, thought this might be a place where I could get an answer especially since I presume it to involve complex physics. Thanks in advance to anyone who helps

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So basically when you can determine the volume of the gas you can get enough of that gas, and eventually it’ll be able to be weighed then you just run the calculation in reverse weight divided by volume(atoms) = atomic weight.

Kind of a weird way of doing things but at a point that’s kind of how it has to be done.

If you know the density of helium gas at a fixed temperature and pressure, which is the normal kind of fussy to work out (ie, careful measurements and lots of mistakes), you can apply the [ideal gas law](https://en.wikipedia.org/wiki/Ideal_gas_law) to calculate the number of helium particles per gram, which is what you’re looking for. And, if I remember my chemistry correctly, you’ll get lucky, because helium obeys the ideal gas law better than some other, more familiar gasses.