Re: "archaic" view of atom structure

From: Lewis Mammel (l.mammel_at_worldnet.att.net)
Date: 12/17/04 

Date: Fri, 17 Dec 2004 03:46:13 GMT

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.

The ground state of the Hydrogen atom is an " l = 0 " state,
meaning there is zero angular momentum of the electron
motion. It is not "orbiting" at all! It is as close to
being at rest as it's possible to get.

The only thing preventing the electron from falling into
the mucleus is the Uncertainty principle. Confining it to
a small volume around the nucleus means the uncertainty in
its position is small, and therefore the uncertainty in
its momentum must be correspondingly large, according to

delta x * delta p > h ( Planck's constant )

"delta x" and "delta p" represent the uncertainty in position
and momentum. This means that there is going to be random motion,
which raises the average kinetic energy.

The actual size of the "orbit" is determined by a balance point
between the lowered energy obtained by getting the negative
electron as close ( on average ) as possible to the positive
nucleus, and the elevated energy caused by the random motion.

The Helium ground state has two electrons in this most
simple "orbit", called "1s". Then with Lithium and Berylium
we add two more electrons to another l=0 ( zero angular momentum )
ground state, giving Berylium a "1s2 2s2" configuration, meaning
2 electrons in the 1s orbital and 2 electrons in the 2s orbital.
None of these electrons have any circular motion. The spatial
distribution of the probabilty functions are spherical blobs,
since there is no axis of motion.

It's only with the fifth electron for Boron that we get a
correspondence to a circular orbit with the "2p" orbital.
The 2p electron can be found to be circling either to the
right or to the left about any axis, always with exactly
one quantum of angular momentum.

Just so you don't feel too bad, note that Bohr assumed
that the electron had to have non-zero angular momentum:

"In the Bohr theory, angular momentum = 0 meant the
so-called pendulum oscillation orbit, in which the
electron would have had to go through the nucleus,
and this was excluded as impossible." ( Herzberg )

Lew Mammel, Jr. ( Ph. D. )

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