The physicist who first proposed the nuclear model of the atom was Ernest Rutherford. In 1911, Rutherford conducted a series of experiments, known as the gold foil experiment, in which he directed alpha particles at a thin sheet of gold. He observed that most of the particles passed through the foil, but some were deflected at large angles, and a few even bounced back. This led him to conclude that the atom is mostly empty space, with a small, dense nucleus at its center that contains positively charged protons.