In the Franck–Hertz experiment, what property of atoms did the results demonstrate?

The Franck–Hertz experiment demonstrated that atoms have quantized energy levels.

James Franck and Gustav Hertz reported the experiment in 1914. They sent electrons through low-pressure mercury vapor and measured the current reaching a collector. As the accelerating voltage increased, the current repeatedly dropped at particular voltage intervals rather than changing smoothly.

The drops occurred because electrons lost specific amounts of energy in collisions with mercury atoms. Those losses excited the mercury atoms from one allowed state to another. The results provided direct experimental support for the emerging quantum theory of atomic structure.

The experiment is often confused with a measurement of the electron’s charge or with the photoelectric effect. Instead, it showed that atoms absorb energy in discrete amounts. Franck and Hertz were awarded the 1925 Nobel Prize in Physics for their discovery of the laws governing the impact of an electron upon an atom.