Thomson concluded that the negatively charged particle of cathode rays must be a fundamental part of matter itself. His model presented the atom containing a large number of smaller bodies, which he still called corpuscles. Since common atoms are electrically neutral, Thomson proposed that the atom comprises separate negative and positive parts. The negative corpuscles (electrons) were the carriers of negative electrical charge, and the positive ion left behind was a bubble with much larger mass than the negative corpuscles.
At a time before the discovery of the atomic nucleus, Thomson imagined that “the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification.”
Thomson’s 1904 model was compared by his fellow scientists to a British dessert called plum pudding (Figure 55): the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons’ negative charges, like negatively charged “plums” surrounded by positively charged “pudding.”
Figure 55 In Thomson’s “plum pudding” model of the atom, tiny electron “plums” float inside a much larger blob of positively charged “pudding.” Ernest Rutherford proved this model wrong in 1911 after he discovered the atomic nucleus.
This model was shown to be false by an experiment conducted by Ernest Rutherford in 1911, in which Rutherford discovered that the small nucleus of the atom contained a very high positive charge; this discovery led to the Rutherford model of the atom.
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