We all know Avogadro’s number as 6.022*10^23 but why is that number significant? Why didn’t he choose something more round like 1*10^23 or something to that effect? What is the real world significance of 6.022*10^23?
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**Avogadro’s number relates the quantity of a substance to its molecular weight**: for example, the molecular weight of water is about 18 amu (atomic mass units), so one mole of water weighs about 18 grams. And it turns out there are 6.022*10^23 molecules in 18 grams of water.
Basing the mole on amu is a way to ensure we’re comparing apples to apples.
Avagadros number is the exact number of atoms in a mole which is equivalent to exactly the number of atoms in 12 g of Carbon 12. Avagadro didn’t choose a number, he just figured out what it was. Your question is like asking why is pi pi? Just cause that’s the answer
Human knowledge went from top down. We defined a system of weights and measures – a fairly arbitrary notion of what 1 kg meant and then what 1 g meant to us humans. This is not universal in any sense.
From there, the question was, how many atoms of hydrogen (at the time the limit of our precision) was in 1 gm of hydrogen? And the answer is basically Avogadro’s number. This number is a result (sort of a back derived) of our choice of measure of mass. This is the reason it isn’t some nice round number – we chose our measure of mass before we understood atoms rather than the other way around. Avogadro’s number only makes sense within the SI units of measure – it is also not universal, it is human defined constant.
If we wanted to force this to be a nice number then we’d have to change our definition of the mass of 1g. For most human beings, the inconvenience of redefining masses is hardly worth the “benefit” of a nice round Avogadro’s constant since masses are used for a lot of other useful stuff like trade and economy.
As it turns out, as our measuring precision increased and knowledge of physics increased, even the original definition is somewhat flawed. Avogadro’s number is now more or less the number of protons (not hydrogen atoms) that weigh 1 g. (The actual history is a bit more complex – involving Carbon12 etc but a bit beyond ELI5)
Avogadro did not chose this number, he measured it. The number is how many proton/neutron there are in a gram of material. If he were still on an imperial measurement the number would be different.
I like how an ELI5 starts with “we all know <insert complex number most people never heard of>”
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