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Modern
Atomic Theory:
Models
In
1897, J.J. Thomson discovered the electron
by experimenting with a Crookes, or cathode ray, tube. He
demonstrated that cathode rays were negatively charged. In addition, he
also
studied positively charged particles in neon gas. Thomson realized that
the
accepted model of an atom did not account for negatively or positively
charged
particles. Therefore, he proposed a model of the atom which he likened
to plum
pudding. The negative electrons represented the raisins in the pudding
and the
dough contained the positive charge. Thomson's model of the atom did
explain
some of the electrical properties of the atom due to the electrons, but
failed
to recognize the positive charges in the atom as particles.
In 1911, Ernest Rutherford, a
former student of J.J.
Thomson, proved Thomson's plum pudding structure incorrect. Rutherford
with the assistance of Ernest Marsden and Hans Geiger performed a
series of
experiments using alpha particles. Rutherford
aimed
alpha particles at solid substances such as gold foil and recorded the
location
of the alpha particle "strikes" on a fluorescent screen as they
passed through the foil. To the experimenters’ amazement,
although most of the
alpha particles passed unaffected through the gold foil as expected, a
small
number of particles were deflected at an angle, and a few ricocheted
straight back.
Rutherford
concluded that the atom consisted
of a small, dense, positively charged nucleus in the center of the atom
with
negatively charged electrons surrounding it. The discovery of the
nucleus is
considered to be Rutherford's
greatest
scientific work.
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