Along with Hans Geiger and Ernest Marsden, Rutherford  fired alpha particles at a thin piece of gold foil. They discovered that most of the particles traveled straight through the foil, but a few returned on the same side that they came from. The alpha particles had run into huge concentrations of positive charge and were repelled. Thus proving that in the center of the atom was a dense, positively charged "ball". (the nucleus)

Gold Foil Experiment