1.
Plum Pudding Atoms
In
1897 J.J. Thomson discovered the electron, a negatively
charged particle more than two thousand times lighter
than a hydrogen atom.
Thomson
originally believed that the hydrogen atom must be made
up of more than two thousand electrons, to account for
its mass.
An
atom made of thousands of electrons would have a very
high, negative electric charge. This was not observed,
as atoms are usually uncharged. In 1906 Thomson suggested
that atoms contained far fewer electrons, a number roughly
equal to the atomic number. This is only one electron
in the case of hydrogen, far fewer than the thousands
originally suggested.
These
electrons must have been balanced by some sort of positive
charge. The distribution of charge and mass in the atom
was unknown. Thomson proposed a 'plum pudding' model,
with positive and negative charge filling a sphere only
one ten billionth of a metre across.
This
plum pudding model was generally accepted. Even Thomson's
student Rutherford, who would later prove the model
incorrect, believed in it at the time.
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