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[Phys-L] Re: Sizes of atoms



On 07/06/05 19:51, Ludwik Kowalski wrote:
The average volume per atom can be calculated from the Avogadro number.

That's entirely true, but not helpful for teaching the
central conceptual point.

The last time I checked, Avogadro's number was _defined_
to be the number of atoms in 12 grams of 12C. So if
we didn't already have a way of counting atoms (perhaps
by weight, or by size, or direct enumeration, ...) we
wouldn't have any clue about the value of Avogadro's
number.

Yes, I can look up Avogadro's number in a book. But I
can also just look up the size of atoms in a book.

What I'm looking for is a hands-on, compelling demo
that can be done in high school classes (no electron
microscopes, no neutron scattering, etc.) and imparts
a good "feel" for how big atoms are. It's not an
easy question; there were a lot of very smart
physicists who lived pre-1900, and they didn't figure
it out.

Actually, just looking out the window and observing
that the sky is blue (on a clear day) suffices to get
you in the right ballpark. The scattering of skylight
depends on density fluctuations, which in turn depend
on how many particles there are. If the fluctuations
were much less, the clear-day sky would be black, and
if they were much more, the clear-day sky would be white.

http://www.av8n.com/physics/blue-sky.htm
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