Discovery of the Nucleus: Geiger and Marsden conducted an experiment called the 'gold foil experiment'. They fired alpha particles at a sheet of gold foil. Since protons repel the alpha particles, they sued this to determine where protons are in an atom. Because most alpha particles passed straight through the sheet without being repelled at all, they concluded that the atom must have a lot of space in it. Because some particles were knocked off course, they concluded that the protons were in the center of each atom in a ball of concentrated positive charge, or the nucleus. Finally, since only a very small few were completely reflected back the way they came, they deducted that the nucleus is very small.