In the 1913 Franck–Hertz experiment, what did collisions between electrons and mercury atoms reveal about atomic energy?

The 1913 Franck–Hertz experiment revealed that atoms possess quantized energy levels.

James Franck and Gustav Hertz sent electrons through mercury vapor while varying the electrons’ accelerating voltage. The measured current did not increase smoothly. Instead, it dropped sharply at particular voltages, especially near 4.9 volts, because electrons had transferred a fixed amount of energy to mercury atoms during inelastic collisions.

The mercury atoms used that energy to move from a lower energy state to an excited state. They later emitted ultraviolet light as they returned to a lower state. The discrete energy loss showed that atoms cannot absorb arbitrary amounts of energy; they change between specific allowed states.

The experiment provided strong evidence for the quantized atomic model. It did not prove that electrons disappear during collisions, nor did it measure nuclear fission, which concerns atomic nuclei rather than electron excitation.