Ernest Rutherford discovered the atomic nucleus through a series of experiments involving the scattering of alpha particles. In 1909, Hans Geiger and Ernest Marsden, working under Rutherford, directed a beam of alpha particles (helium nuclei) at a very thin sheet of gold foil. According to the then-accepted "plum pudding" model by J.J. Thomson, the positive charge in atoms was thought to be spread out diffusely, so alpha particles were expected to pass through with only slight deflections. However, the experiment revealed surprising results: while most alpha particles passed straight through the foil, a small fraction were deflected at very large angles, with some even bouncing back toward the source. Rutherford concluded that such large deflections could only be caused by a very dense and positively charged core within the atom, which he called the nucleus. This nucleus contained most of the atom's mass and occupied only a tiny fraction of the atom's volume, implying that the atom was mostly empty space

. Rutherford proposed that electrons orbited this small, dense nucleus much like planets orbit the sun. His calculations estimated the nucleus to be about 1/100,000th the size of the atom

. This discovery fundamentally changed the understanding of atomic structure, replacing the plum pudding model with the nuclear model of the atom and laying the groundwork for the development of modern atomic physics