Re: "archaic" view of atom structure

From: Gregory L. Hansen (glhansen_at_steel.ucs.indiana.edu)
Date: 12/17/04 

Date: Fri, 17 Dec 2004 16:00:55 +0000 (UTC)

In article <1103248811.738935.193980@c13g2000cwb.googlegroups.com>,
 <lindas4@aol.com> wrote:
>Earlier today, I was told that the view of an atom as a nucleus with
>electrons in circular orbits was "archaic". As I understand it (I am
>NOT a "card carrying physicist") this is, indeed a good, simplified
>view of the basic structure, and thus is useful for teaching and for
>calculations. In fact, this structure does occur. For example, it is
>present in hydrogen and helium atoms. In atoms with a larger number of
>electrons, some electrons are forced into larger circular orbits, and
>some into elliptical orbits. Am I OK so far?
>
>Also, as I understand it, the term "cloud" when used to refer to
>electrons and their behavior, simply refers to the fact that the
>electron, unlike a planet in the solar system, does not remain
>constantly in the same orbit, but moves away from and toward the
>nucleus as it travels around it (we all learned in elementary school
>science that heat will cause this to occur). Thus, the electron would
>look, to an observer, like a fuzzy cloud, and not like a clearly
>defined ring.
>
>Would a real "card carrying physicist", hopefully a physics instructor,
>please tell me if I am missing something here?
>

To give maybe a slightly different slant on the answers you've been
getting, it's well known from electromagnetic theory that an accelerated
charged particle, like an electron orbiting something, will radiate. It
can be shown that for a classical picture of the atom, the energy
will be radiated away in a tiny fraction of a second and the electron will
fall into the nucleus. So the old question, why doesn't that happen?

The answer is the electron has already radiated away all the energy it
can, it's fallen as far into the nucleus as it can go. The size of the
orbital is as small as the Heisenberg uncertainty principle will let it
get-- if you relate orbital diameter to momentum using the uncertainty
principle you'll get the Bohr radius, which is the correct second moment
(the standard deviation of position) of the electron wavefunction. 

-- 
"And don't skimp on the mayonnaise!"

Relevant Pages

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  • johnreed take 25 - Part-2 - Modified September 30, 2008
    ... So we built the 3D atom from 3D charged objects in equilibrium, ... of mass, momentum, and energy. ... Led by J.J. Thompson's isolation of an emitted electron and supported ... terms of a standard unit we call charge. ...
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