I think the link given by Gary provides with the most straightforward and convincing demonstration for the classroom (at least for estimation the length of a chain molecule.) Can the same method be used for estimation of the atomic size or at least that of a diatomic molecule? I think yes, even though the corresponding experiment would become more sophisticated.
There are more possibilities, probably also known before Einstein's work on Brownian motion and fluctuations.
For instance, in the Van-der-Vaals equation of state
(P + a/V^2)(V-b) = RT
the constant a describes the Van der Vaals long-range attraction force, and b is just the combined volume of all the atoms in a given mole of gas. Is it possible that for some gases a few isotherms P(V) can be obtained experimentally for a few different respective T not very far above the critical point by the students in a school Lab? If so, then it would be very instructive for the students to find a and b fitting the experimental curves the best, and then from b and Avogadro's number estimate the size of the gas molecule. Also, this size can be estimated from
(sigma) x n x l = 1
where sigma is cross-sectional area of a molecule or atom, n is concentration, and l is the average free path. The latter can be found, e.g. from the gas viscosity. But all this requires that the students must already have sufficient preliminary knowledge of gas theory, and of course, is much less straightforward.
Moses Fayngold
NJIT
-----Original Message-----
From: Forum for Physics Educators on behalf of G.D. Williams
Sent: Sat 7/9/2005 4:42 AM
To: PHYS-L@LISTS.NAU.EDU
Cc:
Subject: Re: Sizes of atoms (was evidence for non-classical behavior)
This used to be a fairly standard experiment in the UK for 11-14 year olds,
doesn't seem that popular these days. There are detailed instructions at:
I found that the trays used need to be as clean as possible and I prefer to
use chalk dust or talc as I have a suspicion that it was inhaling lycopodium
spores that triggered my asthma.
Even if you don't try and convince pupils about the lining up of the
molecules and the monolayer business it still gives a nice upper limit. I
don't know the origin of the experiment but I note that in "Physics for the
Enquiring Mind" by Eric Rogers it says late 19th century and I also recall
reading something about Benjamin Franklin realising that oil on water would
make a very thin layer but not realising the next step, not surprising
really when the nature of the atom was still unknown.
Cheers
Gary
G. D. Williams,
Editor, Physics Education
National Coordinator, IoP Teacher Network
gary.williams@iop.org
gdwilliams100@hotmail.com
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