This document discusses the discoveries of small particles that make up atoms. It describes John Dalton's atomic theory that atoms are the smallest particles that can engage in chemical reactions. It then summarizes J.J. Thompson's discovery of electrons through discharge tube experiments and their role in atoms. Rutherford discovered protons through gold foil experiments and that atoms have a tiny, dense nucleus at their center surrounded by electrons. Rutherford proposed a solar system model of the atom with electrons orbiting the nucleus.