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From: "Fayngold, Moses" <fayngold@ADM.NJIT.EDU>_______________________________________________
Reply-To: Forum for Physics Educators <PHYS-L@list1.ucc.nau.edu>
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: Sizes of atoms (was evidence for non-classical behavior)
Date: Fri, 08 Jul 2005 23:11:29 -0400
I think one of the simplest effects demonstrating the atomic size is the
spread of a droplet on an adhesive substrate. A well known example is the
spread of an oil drop on the water surface. Generally, a chain molecule of
an organic material may stand vertically on the water surface, with its
hydrophilic end submerged and hydrophobic one sticking out. Assuming that a
drop of such material will spread until it flattens down to the monolayer,
we can find its thickness as the ratio of the initial volume to the area of
the formed spot. The spot may be visible due to change of the optical
properties of the surface covered by it. It works for some organic
materials with long molecules provided we have a large enough surface of
still water available. Unfortunately, I do not remember the source
describing the details. But in principle, this method could be applied for
estimation of the atomic size as well in experiments with microdroplets put
onto an appropriate substrate. But this may be difficult to show as a l
I also agree with Jack that another straightforward demonstration comes
from the scattering experiments. For estimation of the atomic size the
scattering in the two oncoming low-energy atomic beams may be relevant. I
disagree, however, that a sharply defined surface will result in the rapid
fall-off in the corresponding diffraction peak. Quite the contrary, in the
Fraunhofer-type diffraction corresponding to the scattering experiments,
the sharper the surface (all other things equal), the wider the peak, and
vice versa. In a special case, when the surface fuzziness is Haussian, so
is the diffraction peak, but their respective widths are, again,
reciprocal. The nature of this relationship is the same as the
indeterminacy relationship between the position and momentum in Quantum
Mechanics.
Moses Fayngold,
NJIT
-----Original Message-----
From: Forum for Physics Educators on behalf of Jack Uretsky
Sent: Fri 7/8/2005 1:55 PM
To: PHYS-L@LISTS.NAU.EDU
Cc:
Subject: Re: Sizes of atoms (was evidence for non-classical
behavior)
Hi all-
I disagree strongly with Joe. One of the great surprises to me
bout atoms and nuclei is the extent to which we can think about them as
classical systems - but with appropriate modifications.
The first clue comes from looking at atomic wave functions and
noting that they involve deecreasing exponentials. I think of decaying
exponentials as quantum mechanics' replacement for classical sharp
boundaries.
The second clue comes from looking at the ratios (a word that
many of your students don't understand) of atomic "weights" to densities
of elemental substances. With a few marked exceptions, these are
surprisingly (to me) constant. This fact tells me that most atoms are
about the same "size".
Sizes of atomic and sub-atomic objects are typically measured in
scattering experiments, with sizes given, e.g., by diffraction peaks. For
electron scattering on noble gases see, e.g. (again), the graphs in
Purcell, et al., Phys Rev A 3 (1971) 508.
Rapid fall-off from a diffraction peak correspond to "abrupt"
termination of a sub-atomic object. That's what we observe in almost all
cases.
Regards,
Jack
On Wed, 6 Jul 2005, jbellina wrote:
Since these are not particles as we usually think of them, they don'tof
have a well defined size...it isn't that they occupy some space that
termininates abruptly. Size is a macroscopic concept that seems to
have little precise meaning microscopically.
As pointed out diffraction techniques give information about periodic
spacings, not size. Even something like scanning tunnelling microscopy
does not show atom size, if such a term were really meaningful, rather
it shows the spacial distribution of electron density.
Of course when one thinks that way, real objects don't terminate
abruptly, they all have some sort of roughness with hills and valleys,
but I think that is a different issue than the really microscopic one.
cheers,
joe
On Jul 6, 2005, at 2:14 PM, Ludwik Kowalski wrote:
On Wednesday, Jul 6, 2005, at 14:15 America/New_York, Bernard Cleyet
wrote:
. . . I must be msng. something here, as I thought booth LEED
and old ordinary X-ray diff. would do this. . . .
Is it not true that X-ray crystalography gives as distances between
scattering centers (atoms) and not sizes of atoms? To estimate sizes
atoms one can assume that (in solids) they "essentially touch eachJoseph J. Bellina, Jr. Ph.D.
others."
The size of an atomic nucleus can be operationally defined in terms of
ranges of nuclear forces. Can the size of an atom be operationally
defined in the same way?
Ludwik Kowalski
Let the perfect not be the enemy of the good.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley