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How did Rutherford’s gold-foil experiment reveal the atomic nucleus?

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Bohr model diagram of a hydrogen atom showing a green nucleus labeled H, a single electron orbit, and text with atomic mass 1.008 and electron configuration 1. — atom The Bohr model describes the structure of atoms, in this case hydrogen.

Ernest Rutherford’s gold-foil experiment, conducted by Hans Geiger and Ernest Marsden in 1909, fundamentally changed our understanding of atomic structure by disproving the "plum-pudding" model.1

In the experiment, researchers fired a beam of alpha particles—positively charged, massive particles—at a thin sheet of gold foil. While most particles passed straight through, implying that atoms consist largely of empty space, a small fraction were deflected at large angles, and some even bounced directly back toward the source.21

Rutherford concluded that these results could only occur if the atom contained a tiny, dense, and positively charged core, which he termed the nucleus. Because the alpha particles were positively charged, they experienced strong electrostatic repulsion when approaching this concentrated positive center. This discovery revealed that nearly all of an atom's mass is concentrated in this minute nucleus, while negatively charged electrons orbit at a great distance, creating a planetary-like system.13