Model of Bohr Model
Date: Thursday, November 28, 2002
Every year I have my 8th grade science students build a
3D atom model of their choice. They must show electrons in their correct
energy level and a nucleus made up of protons and neutrons. This is a
Bohr type model built mainly to acquaint the student with the 3 basic
subatomic particles. Although this is not a scale model I suggest the
electrons be smaller than neutrons and protons. A father claiming to be
a physicist claims that I cannot model the neutrons and protons as
separate particles. He insists that the models can only show a nucleus
with the numbers of protons and neutrons marked on it. Am I making a
major atomic mistake here?
His assertion is news to me. I have always addressed them as distinct
nucleons. Is he suggesting that neutrons and protons are somehow being
"formed" (for example) at the instant a radioactive nucleus decays?
Regarding scale: In so far as diameter (not mass) is concerned, electrons
are much larger than either protons or neutrons.
Technically, the father is correct. If we could stand inside the nucleus, we
could not say the proton is over there to the right, the neutron is down,
and so on. Protons and neutrons are composite particles consisting of
quarks, gluons, and other sub-atomic "goblins". HOWEVER, THAT IS NOT THE
OBJECTIVE OF YOUR TEACHING, AND YOU ARE NOT MAKING A MAJOR MISTAKE IN YOUR
You are teaching the basics of atomic theory. The positively charged "core"
of the problem is that the mass of an atom is concentrated in a very small
space and the negatively charged electrons are "smeared" over a much larger
volume. FYI the classical radius of a FREE electron is 2.817940285x10^-15
meters = e^2/mc^2, but within an atom it loses this "particle" aspect and
behaves as though it is a standing wave, but even this is too advanced for
your audience (probably, unless you have some gifted kids who are going to
challenge you for a more detailed MODEL of the atom -- that's a bigger,
The Bohr model has a spell-binding history, and was a major advance in
physics of the early 20th century. It does account for some very critical
experimental data -- it explains the visible-UV spectrum of a H atom (at
least the major part). Even here high resolution spectra showed some
multiple lines, rather than single lines predicted correctly by the Bohr
model. The Bohr model had to be modified and so on.
You have to teach your students tinker toys before you train them to be
architects. I think you are right on target. The many subtleties are for a
later course on atomic physics.
If you want to be perfect, you cannot model the electrons in circular
orbits. Consider what you are trying to show before you evaluate your
A "real" atom has never been seen. Electron microscopes can "see" electric
fields around atoms. Perhaps they can see electric fields around nuclei.
They do not see the atom in the strictest sense. If you want to show a real
atom, you need a "cloud" of protons with a variety of oscillations: out and
in, hot dog-shape and hamburger-shape, .... You need these oscillating
clouds for both protons and neutrons. The clouds are all centered in the
same place, the center of the nucleus. There is no exact border. As you
get further from the nucleus' center, it just gets more unlikely to find a
proton or neutron at any time. Still, it is not completely impossible.
Electrons also orbit in clouds around the nucleus. Each has its own
distribution. Some can be just about anywhere. Some have preferred
locations: rings, figure-8 shaped orbits, at least four different sets. It
is possible for an electron to spend a little time passing through the
nucleus. The AVERAGE distance is very far out. That is where the rings
If you show the protons and neutrons arranged in a compact ball-shape, you
are not actually that far off. Something like a standard crystal structure
works. Just do not make a circle of 8 protons on top of a circle of 8
neutrons for oxygen. A better shape for the nucleus is a solid sphere.
Carbon balls in the pattern of coal would work quite well.
As for size, you cannot be accurate in a visual model. A common description
is the following: a nucleus is a pea at the center of a football stadium
with the electrons at the goal posts. A visual model does not try to show
that. It usually tries to show that the electrons are separate from the
nucleus and that the number of protons match the number of electrons. You
can also use it to show why you need more neutrons in larger atoms. Too
many protons "in contact" with each other would blow the atom apart. The
neutrons help hold the protons together.
Dr. Ken Mellendorf
Illinois Central College
You are doing fine. Everybody's idealized view of a nucleus is of a
collection of neutrons and protons, and this picture contains the first
information one should have about a nucleus. If I were to refine this
model of a nucleus, the first thing I would think about would be
subcollections of nucleons within a nucleus, e.g., alpha particles. The
last thing I would think about doing is
erasing all details of nuclear structure and replacing this visual
information with numbers.
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Update: June 2012