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The discovery of the electron had only increased scientists’
curiosity about the structure of the atom. After Thomsen’s finding, scientists
continued to explore the secrets of the atom.
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| Ernest Rutherford was observing the effects of
shooting a narrow beam of small alpha particles at a thin gold foil. Rutherford
noticed when the alpha particles struck the thin metal, some of them scattered instead of
continuing straight through. |
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ALPHA
PARTICLE
A helium nucleus; often used as a projectile to
shoot at other atoms |
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 The
discovery occurred when Hans Geiger, one of Rutherford’s assistants noticed that the
number of alpha particles scattered by an angle greater than 10 degrees was much more
than predicted.
| Rutherford wrote: |
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"I had observed the scattering of alpha particles, and... it
was as if you had fired a 15-inch naval shell at a piece of tissue paper and the shell
came right back and hit you... it was then that I had an idea of an atom with a minute
massive centre, carrying a charge."
Source: Rutherford et al., Project Physics Unit 5 Text, 1971
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From Rutherford’s discovery came the realization of the
idea of the nucleus – a small, dense concentration of charge and mass.
Rutherford explained that this was possible because any particle that ran into the nucleus
would rebound and get deflected, which would not happen if a particle had simply traveled
through an atom "cloud." Dr. Geiger led an experiment to verify these
conclusions and proved Rutherford correct.
Rutherford’s discovery set the stage for Bohr’s orbital model of the atom,
which utilized the idea of a central nucleus.
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