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Plum Pudding Atoms
In 1897, scientist J.J. Thomson discovered the electron. To explain the balance between negative and positive charges in an atom, he proposed that an atom was like "plum pudding." In this model, negatively-charged electrons floated freely in a "blob" of positive charge—like raisins in a plum pudding. A few years later, a scientist named Ernest Rutherford, a student of Thomson's, was conducting an experiment in which he bounced alpha particles off a sheet of gold foil. The particles were deflected off the gold atoms at large angles. This experiment showed that the positive charge had to be concentrated in a small mass—not distributed evenly with negative charges as the "plum pudding" model had suggested. Rutherford suggested that the positive charge was collected in a massive nucleus and the lighter electrons orbited the nucleus as planets orbit the Sun.