Around 1900, people thought atoms were permeable
spheres. In 1909, Rutherford, supervising Geiger and Marsden, tested the
validity of this theory in his now-famous gold-foil experiment.
The experiment was quite simple: a radioactive source would shoot a stream of alpha particles at a very thin gold foil (alpha particles have very little mass compared to gold atoms.) Surrounding the gold foil was a screen covered with zinc sulfide so that the alpha particles hitting the screen would leave microscopic spots in the zinc sulfide.
