Final answer:
The positively charged alpha particles bounced back after encountering the dense, positively charged nuclei of gold atoms in the Rutherford gold foil experiment, leading to the discovery of the atomic nucleus.
Step-by-step explanation:
When positively charged alpha particles were radiated onto a gold atom and a few of them bounced back, it was caused by the repulsive electrostatic force from the positive nuclei of the gold atoms. In the Rutherford gold foil experiment, this surprising result led to the discovery that atoms consist of a dense, positively charged nucleus where nearly all mass and positive charge are concentrated. The deflection of the alpha particles, according to Rutherford's model, indicated that the atom is a large space, with most of the mass and the entire positive charge confined to the atomic nucleus. The few positively charged alpha particles that bounced back did so after coming into close contact with or directly hitting the gold atoms' nuclei, which repelled them due to the like-positive charges repelling each other.