Atom
In 1909, Ernest Rutherford used the Geiger–Marsden experiment to prove that most of an atom is in a very small space, the atomic nucleus. Rutherford took a photo plate and covered it with gold foil. He then shot alpha particles (made of two protons and two neutrons stuck together) at it. Many of the particles went through the gold foil, which proved that atoms are mostly empty space. Electrons are so small and fast-moving that they did not block the particles from going through. Rutherford later discovered protons in the nucleus.