Well, when the alpha particles bombarded the gold foil, a lot fo them were deflected at unpredicted angles (as in almost directly back at the source in the lead container). This meant that the Alpha Particles had a charge that was the same as the Gold Particles (they both turned out to be positive, since electrons are not in the nucleus) since they did not attract, and there had to be some kind of particle in the gold foil that is big enough to deflect the tiny alpha particles (helium-4 atoms).
