In the 1909 Geiger–Marsden experiment, what did the small number of sharply deflected alpha particles reveal?

The 1909 Geiger–Marsden experiment revealed that atoms contain a small, dense atomic nucleus.

Hans Geiger and Ernest Marsden fired alpha particles at thin gold foil under Ernest Rutherford’s supervision. Most particles passed through with little deflection, but a tiny fraction bounced away at very large angles, with a few returning nearly backward.

The rare large deflections were extremely difficult to explain with J. J. Thomson’s diffuse “plum pudding” model. Rutherford concluded that nearly all the atom’s positive charge and mass is concentrated in a compact central region, while most of the atom is empty space.

The experiment is often called Rutherford’s gold-foil experiment, although the actual scattering work was performed by Geiger and Marsden. Rutherford proposed the nuclear model in 1911.