Rutherford had a sheet of gold foil just a few atoms thick, and surrounded it with detectors. He then fired alpha particles at the gold sheet, knowing that these were positively charged. Some of these were deflected from their paths, a very small number were reflected, but most passed straight through the foil. This told Rutherford that most of the atom must be empty space. He explained the deflections and reflections with a concentration of charge in the middle of the space of each atom. As so few were reflected and deflected compared to those that went straight through, the charge must be tiny compared to the atom. As it was repelling the positive alpha particles, it must also be positive. This was the nucleus.