The Birth of Quantum Mechanics

In 1909 and 1911, working at the University of Manchester, Ernest Rutherford directed an experiment, carried out by Hans Geiger and Ernest Marsden, in which alpha particles were fired at a thin sheet of gold foil. Most particles passed straight through, but a small fraction bounced back at sharp angles, a result Rutherford said was "almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." He concluded that an atom's positive charge and nearly all its mass are concentrated in a tiny, dense central nucleus, replacing Thomson's plum-pudding model and effectively founding nuclear physics.