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Rutherford model
Excerpted from Wikipedia, on July 8, 2010
(Background: an alpha particle [α particle] has two protons, two neutrons, and no
electrons. They are naturally occurring particles that result from radioactive decay).
The Rutherford model or planetary model is a model of the atom devised by Ernest
Rutherford. Rutherford directed the famous Geiger-Marsden experiment in 1909, which
suggested on Rutherford's 1911 analysis that the so-called "plum pudding model" of J. J.
Thomson of the atom was incorrect. Rutherford's new model for the atom, based on the
experimental results, had the new features of a relatively high central charge concentrated
into a very small volume in comparison to the rest of the atom and containing the bulk of
the atomic mass (the nucleus of the atom).
Rutherford's model did not make any new headway in explaining the electron-structure of
the atom; in this regard Rutherford merely mentioned earlier atomic models in which a
number of tiny electrons circled the nucleus like planets around the sun, or a ring around
a planet (such as Saturn). However, by implication, Rutherford's concentration of most of
the atom's mass into a very small core made a planetary model an even more likely
metaphor than before, as such a core would contain most of the atom's mass, in an
analogous way to the Sun containing most of the solar system's mass.
Experimental basis for the model
In Rutherford's gold foil experiment conducted by his students Hans Geiger and Ernest
Marsden, a narrow beam of alpha particles was established, passing through very thin (a
few hundred atoms thick) gold foil. The alpha particles were detected by a zinc sulfide
screen, which emits a flash of light upon an alpha particle collision. Rutherford
hypothesized that, assuming the then-current "plum pudding" model of the atom was
correct, the positively charged alpha particles would be only slightly deflected, if at all,
by the dispersed positive charge predicted.
It was found that some of the alpha particles were deflected at much larger angles than
expected and some even bounced almost directly back. Although most of the alpha
particles went straight through as expected, Rutherford commented that the few particles
that were deflected was akin to shooting a fifteen inch shell at tissue paper only to have it
bounce off, again assuming the "plum pudding" theory was correct. It was determined
that the atom's positive charge was concentrated in a small area in its center, making the
positive charge dense enough to deflect any positively charged alpha particles that came
close to what was later termed the nucleus. Note: it was not known at the time that alpha
particles were themselves nuclei nor was the existence of protons or neutrons known.
The gold foil experiment
Top: Expected results: alpha particles
passing through the plum pudding model of
the atom with negligible deflection.
Bottom: Observed results: a small portion of
the particles were deflected by the
concentrated positive charge of the nucleus
In 1911, Rutherford came forth with his own physical model for subatomic structure, as
an interpretation for the unexpected experimental results. In it, the atom is made up of a
central charge (this is the modern atomic nucleus, though Rutherford did not use the term
"nucleus" in his paper) surrounded by a cloud of (presumably) orbiting electrons. In this
1911 paper, Rutherford only commits himself to a small central region of very high
positive or negative charge in the atom.
"For concreteness, consider the passage of a high speed α particle through an atom
having a positive central charge N e, and surrounded by a compensating charge of N
electrons."
From purely energetic considerations of how far alpha particles of known speed would be
able to penetrate toward a central charge of 100 e, Rutherford was able to calculate that
the radius of his gold central charge would need to be less (how much less could not be
told) than 3.4 x 10-14 metres (the modern value is only about a fifth of this). This was in a
gold atom known to be 10-10 metres or so in radius -- a very surprising finding, as it
implied a strong central charge less than 1/3000th of the diameter of the atom.
The Rutherford model served to concentrate a great deal of the atom's charge and mass to
a very small core, but didn't attribute any structure to the remaining electrons and
remaining atomic volume. It did mention the atomic model of Hantaro Nagaoka, in which
the electrons are arranged in one or more rings, with the specific metaphorical structure
of the stable rings of Saturn. The so-called plum pudding model of J. J. Thomson had
also had rings of orbiting electrons.
The Rutherford paper suggested that the central charge of an atom might be
"proportional" to its atomic mass in hydrogen mass units u (roughly 1/2 of it, in
Rutherford's model). For gold, this mass number is 197 (not then known to great
accuracy) and was therefore modeled by Rutherford to be possibly 196 u.
Contribution to modern science
After Rutherford's discovery, scientists started to realize that the atom is not ultimately a
single particle, but is made up of far smaller subatomic particles. Following research was
done to figure out the exact atomic structure which led to Rutherford’s gold foil
experiment. They eventually discovered that atoms have a positively-charged nucleus.
Since electrons were found to be even smaller, this meant that the atom consists of mostly
empty space.
Symbolism
Shield of the U.S. Atomic Energy Commission.
Despite its inaccuracy, the Rutherford model caught the imagination of the public in a
way that the more correct Bohr model did not, and has continually been used as a symbol
for atoms and atomic energy. Examples of its use over the past century include:
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The logo of the United States Atomic Energy Commission, which was in part
responsible for its later usage in relation to nuclear fission technology in
particular.
The US minor league baseball Albuquerque Isotopes' logo is a Rutherford atom,
with the electron orbits forming an A.
On maps, it is generally used to indicate a nuclear power installation.