Why did a few alpha particles bounce back from the gold foil?

In the famous gold foil experiment conducted under the direction of Ernest Rutherford, the observation that a very small number of alpha particles bounced back toward the source was critical to understanding atomic structure.1

This phenomenon occurred because the alpha particles—which are positively charged—encountered a tiny, dense, and positively charged nucleus at the center of the atom. Because both the alpha particles and the nucleus possess a positive charge, they exert a strong electrostatic force of repulsion on one another.12

When an alpha particle was aimed directly at this dense, massive core, the repulsive force was sufficient to stop the particle and deflect it backward. This result proved that the atom is not a uniform "plum-pudding" of charge, but rather consists of a concentrated, positively charged nucleus surrounded by mostly empty space, through which the majority of alpha particles passed unimpeded.12