Discovery of subatomic particles: Thomson and the Electron
Atoms were thought to be the smallest possible division of matter until 1897 when J.J. Thomson discovered the electron through his work on cathode rays. Through experimentation, Thomson discovered that the rays could be deflected by an electric field (in addition to magnetic fields, which was already known). He concluded that these rays, rather than being a form of light, were composed of very light negatively charged particles he called "corpuscles" (they would later be renamed electrons by other scientists).

He concluded that atoms were divisible, and that the corpuscles were their building blocks. To explain the overall neutral charge of the atom, he proposed that the corpuscles (electrons)  were distributed in a uniform sea of positive charge; this was the plum pudding model  as the electrons were embedded in the positive charge like plums in a plum pudding (although in Thomson's model they were not stationary).

Why did the Plum Pudding Model Fail?

Rutherford set out to verify the Plum Pudding model in 1907. When Rutherford bombarded a gold foil with alpha particles, he expected that most of the particles will be reflected back from the positive charge sphere of the atom. However, what he noticed was that almost all of them passed through, with only a few deflected back in 180 degrees. This meant that the picture of Plum Pudding model of the atom was inaccurate.


Source: http://en.wikipedia.org/wiki/Atomic_theory and http://www.buzzle.com/articles/plum-pudding-model.html
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