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Why did some alpha particles bounce back during Rutherford’s gold-foil experiment?

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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.

In Ernest Rutherford’s gold-foil experiment, the observation that some alpha particles bounced back toward the source was critical to discovering the atom’s structure.12

While most alpha particles passed straight through the foil—indicating that atoms are composed mostly of empty space—a very small number were deflected at large angles or reflected backward. This occurred because the alpha particles, which are positively charged, encountered a tiny, dense, and positively charged nucleus at the center of the atom.23

Because both the alpha particles and the nucleus carry a positive charge, they exert a strong repulsive electrostatic force on one another. Only a target that was both relatively heavy and highly concentrated with positive charge could account for such a powerful repulsion, causing the particles to recoil. This evidence forced the abandonment of the earlier "plum-pudding" model in favor of the Rutherford model, which correctly identified the nucleus as the center of the atom's mass and charge.234