The 1909 Geiger–Marsden experiment found that a small fraction of alpha particles underwent large-angle deflection.
Hans Geiger and Ernest Marsden, working under Ernest Rutherford, fired alpha particles at thin metal foils and recorded flashes on a zinc sulfide screen. Most particles passed through with little change, but a very small number scattered through large angles, with some apparently rebounding.
The result was unexpected under the prevailing plum-pudding model, which spread positive charge throughout the atom. Rutherford later interpreted the rare strong deflections as evidence that nearly all positive charge and mass are concentrated in a tiny nucleus.
This experiment is closely related to, but not identical with, the often-named Rutherford gold-foil experiment. Geiger and Marsden performed the scattering work in 1909; Rutherford published the nuclear interpretation in 1911. The key observation was not that most particles bounced back, but that rare large deflections occurred.