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| Q: | One of my students asked me, "Why does the electron move at all?" I admitted I didn’t know and would like to find out for myself and for her. Thanks
-David DeCarli
Cromwell High School, CT, USA |
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| A: | David -
Awesome question! (Give your student my compliments for thinking it
up!) Naturally, one would think that because protons are positively
charged, and electrons are negatively charged, the two should attract
and stick together. The reason that doesn’t happen can’t even begin
to be explained using classical physics. This was one of the key
mysteries that were cleared up right away by the invention of quantum
mechanics around 1925.
The picture you often see of electrons as small objects
circling a nucleus in well defined "orbits" is actually quite wrong.
As we now understand it, the electrons aren’t really at any one place
at any time at all. Instead they exist as a sort of cloud. The cloud
can compress to a very small space briefly if you probe it in the
right way, but before that it really acts like a spread-out cloud.
For example, the electron in a hydrogen atom likes to occupy a
spherical volume surrounding the proton. If you think of the proton
as the size of a grain of salt, then the electron cloud would have
about a ten foot radius. If you probe, you’ll probably find the
electron somewhere in that region.
The weird thing about that cloud is that its spread in
space is related to the spread of possible momenta (or velocities) of
the electron. So here’s the key point, which we won’t pretend to
explain here. The more squashed in the cloud gets, the more spread
out the range of momenta has to get. That’s called Heisenberg’s
uncertainty principle. It could quit moving if it spread out more,
but that would mean not being as near the nucleus, and having higher
potential energy. Big momenta mean big kinetic energies. So the
cloud can lower its potential energy by squishing in closer to the
nucleus, but when it squishes in too far its kinetic energy goes up
more than its potential energy goes down. So it settles at a happy
medium, with the lowest possible energy, and that gives the cloud and
thus the atom its size.
That basically answers your question, although we admit that
the answer sounds strange. There really are very definite
mathematical descriptions to go along with those words.
You might be interested in some more properties of those electrons in atoms.
If just the right amount on energy is applied, it is
possible to knock an electron up to a higher energy orbital (a
different shape of cloud, not so close to the nucleus), or even
completely off of the atom. If electrons are knocked off of the
atoms, they can create electricity. (This is what you see when you
look at a VanDeGraff Generator or at lightning.)
If they are only given some energy, but not enough to
knock them loose, they will move from one orbital to another (say
from the S-orbital to the P-orbital). But if there is no other
electron in the lower-energy orbital, they will fall back down again.
When they do, they release energy in the form of a photon (light).
This is part of the concept that lasers are based on.
Well...I apologize for this answer being so long. Thanks
for sticking with me up to here! I hope this answers your question.
-Tamara
(republished on 07/21/06) |
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