American Chemical Society (ACS) Chemistry Practice Exam 2025 - Free ACS Chemistry Practice Questions and Study Guide

The conclusion that atoms are mostly empty space stems from the observation that most alpha particles passed through the metal foil without any deflection. In Rutherford's gold foil experiment, a source of alpha particles was directed at a very thin sheet of gold foil. If atoms were densely packed with positive charge, as was the prevailing belief at that time, one would expect that the majority of the alpha particles would be deflected at large angles due to repulsion from the dense positive charge of the gold atoms.

However, the fact that most alpha particles passed through the foil undisturbed suggested that there was a significant amount of empty space within the atom. This indicated that the positive charge was concentrated in a very small central region of the atom—the nucleus—while the rest of the atomic structure was largely empty. This pivotal observation led to a new understanding of atomic structure, shifting away from the previous models that depicted atoms as solid spheres.