Teach Astronomy - Rutherford's Experiments

http://www.teachastronomy.com/ Around 1900 atomic structure had been hinted at by experiments by Roentgen and Becquerel, and the physicist J.J. Thompson had isolated the negatively charged and light particle called the electron. But it was the experiments of Ernest Rutherford that cemented the modern view of the atom. His experiments were deceptively simple, but they had profound conclusions. Rutherford took alpha particles, helium nuclei, and fired them at a gold foil. He saw three things. Most of the alpha particles traveled through the gold foil uninterrupted. A small fraction of the alpha particles were deflected by a small angle to the side, and an even smaller fraction came back in the direction of origin. He learned several things from these outcomes. The fact that most alpha particles passed through undeflected told him that the atom was mostly empty space. The fact that some were deflected by small angles was consistent with electrical charge of the atomic nucleus interacting with the positive electric charge of the alpha particle, an electrical repulsion in operation. The final result which for him was a complete surprise showed that the small nucleus of the atom must contain most of the mass of the atom, that being the only reason whereby the lighter alpha particle would rebound completely in the direction of its original motion.