Ernest Rutherford

Rutherford
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In 1910, Ernest Rutherford provided the next great leap in atomic theory with his experiments, which involved firing alpha particles at thin foils of gold. In keeping with Thomson's "plum-pudding" model, Rutherford, along with his associates Hans Geiger and undergraduate Ernest Marsden, expected most alpha particles to pass through the foil with little to no deflection. The results were somewhat shocking. Although most particles passed with little deflection, some deflected at large angles. In some cases, they rebounded in the opposite direction! Rutherford was quoted as saying, "It was as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
                                                                                                                   Gold Experiment
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Rutherford arrived at two different conclusions. First, atoms must be mostly empty space. Second, the atom's positive charges must be concentrated in a dense, central nucleus, which is the core of the atom. Rutherford named these positively charged particles protons. He then proposed the "planetary" model of the atom, which stated that atoms have a dense, positively charged nucleus orbited by negatively charged particles.

Planetary Model
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Nucleus
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