At the dawn of the 20^th century, scientists knew that negatively charged particles existed. Because most atoms have a neutral charge, scientists thought that positive particles might exist to balance the negative particles. They were also curious about how many charged particles were in the atom and how the particles were arranged. Thomson proposed an atomic model in 1904 in response to these curiosities.

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In Thomson’s "Plum Pudding Model" each atom was a sphere filled with a positively charged fluid. The fluid was called the "pudding." Scattered in this fluid were electrons known as the "plums." The radius of the model was 10^-10 meters.

Thomson suggested that the positive fluid held the negative charges, the electrons, in the atom because of electrical forces. However, this was only a very vague explanation and failed to provide any definite answers.

Discovery of the Electron

Millikan's Oil Drop Experiment