In a famous experiment Rutherford bombarded a very thin gold (Au) foil with positively charged alpha particles. They found that most of the alpha particles passed right through the Au foil. However a?

small fraction of the alpha particles were deflected at large angles, and some even bounced straight back. This unexpected result led Rutherford to conclude that the atom consists mostly of empty space, with a dense, positively charged nucleus at its center, which contains most of the atom's mass. This experiment was pivotal in shaping the modern understanding of atomic structure, leading to the nuclear model of the atom.