What unexpected result did Rutherford observe in his gold foil experiment?

In Rutherford's gold foil experiment, the unexpected result observed was that some alpha particles were deflected at large angles. This finding was surprising for several reasons.

Prior to the experiment, the prevailing model of the atom was the plum pudding model, which suggested that positive charge was spread out evenly throughout the atom. If this model were correct, it was expected that alpha particles, which are positively charged, would pass through the gold foil with minimal deflection due to the diffuse positive charge around the electrons. However, Rutherford found that while most alpha particles passed straight through the foil (indicating that the atom is mostly empty space), a small fraction were deflected sharply, sometimes back toward the source.

This indicated that there were regions of concentrated positive charge within the atom, leading to the proposal of a more nuclear model of the atom, where a dense nucleus containing most of the atom's mass and positive charge exists at the center, while electrons orbit this nucleus. Thus, the deflections at large angles were crucial in reshaping scientific understanding of atomic structure at the time.