Editing Franck–Hertz experiment (section)

{{Short description|1914 confirmation of the atom's quantum nature}}
{{Use British English|date=April 2025}}
[[File:FranckHertzHgTube.jpg|thumb|upright=0.5|Photograph of a vacuum tube used for the Franck–Hertz experiment in instructional laboratories. There is a droplet of mercury inside the tube, which is not visible in the photograph. C – cathode assembly; the cathode is hot, and glows orange. It emits electrons which pass through the metal mesh grid (G) and are collected as an electric current by the anode (A). |alt=Photograph of a sealed glass cylinder. Wires penetrate the cylinder at its top, bottom, and side. Three wires lead to a cathode assembly; the top and side wires lead to a disc and a mesh that are close and parallel to each other. The wires are attached to feedthroughs on an aluminum panel in the background.]]
{{Quantum mechanics|cTopic=Experiments}}

The '''Franck–Hertz experiment''' was the first electrical measurement to clearly show the [[History of quantum mechanics|quantum nature of atoms]]. It was presented on April 24, 1914, to the [[Deutsche Physikalische Gesellschaft|German Physical Society]] in a paper by [[James Franck]] and [[Gustav Ludwig Hertz|Gustav Hertz]].<ref name=FH1 /><ref name=Lemmerich /> Franck and Hertz had designed a [[vacuum tube]] for studying energetic [[electron]]s that flew through a thin vapour of [[mercury (element)|mercury]] atoms. They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 [[electron volt]]s) of its [[kinetic energy]] before flying away.<ref name=Pais2 /> This energy loss corresponds to decelerating the electron from a [[speed]] of about 1.3 million metres per second to zero.<ref name=Nuffield /> A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy.

These experimental results proved to be consistent with the [[Bohr model|Bohr model for atoms]] that had been proposed the previous year by [[Niels Bohr]]. The Bohr model was a precursor of [[quantum mechanics]] and of the [[electron shell]] model of atoms. Its key feature was that an electron inside an atom occupies one of the atom's "quantum energy levels".  Before the collision, an electron inside the mercury atom occupies its lowest available energy level. After the collision, the electron inside occupies a higher energy level with 4.9 electronvolts (eV) more energy. This means that the electron is more loosely bound to the mercury atom. There were no intermediate levels or possibilities in Bohr's quantum model. This feature was "revolutionary" because it was inconsistent with the expectation that an electron could be bound to an [[atomic nucleus|atom's nucleus]] by any amount of energy.<ref name=Pais2 /><ref name=Cohen />

In a second paper presented in May 1914, Franck and Hertz reported on the light emission by the mercury atoms that had absorbed energy from collisions.<ref name=FH2 /> They showed that the [[wavelength]] of this [[ultraviolet]] light corresponded exactly to the 4.9 eV of energy that the flying electron had lost. The relationship of energy and wavelength had also been predicted by Bohr because he had followed the structure laid out by [[Hendrik Lorentz]] at the 1911 Solvay Congress. At Solvay, Hendrik Lorentz suggested after Einstein's talk on quantum structure that the energy of a rotator be set equal to ''nhv''.<ref>Original Proceedings of the 1911 Solvay Conference published 1912. THÉORIE DU RAYONNEMENT ET LES QUANTA. RAPPORTS ET DISCUSSIONS DELA Réunion tenue à Bruxelles, du 30 octobre au 3 novembre 1911, Sous les Auspices dk M. E. SOLVAY. Publiés par MM. P. LANGEVIN et M. de BROGLIE. Translated from the French, p. 447.</ref><ref> Heilbron, John L., and Thomas S. Kuhn. “The Genesis of the Bohr Atom.” Historical Studies in the Physical Sciences, vol. 1, University of California Press, 1969, pp. vi–290, p. 244 {{doi|10.2307/27757291}}.</ref> Therefore, Bohr had followed the instructions given in 1911 and copied the formula proposed by Lorentz and others into his 1913 [[Bohr model|atomic model]].<ref>See [[Bohr model]]</ref> Lorentz had been correct. The quantisation of the atoms matched his formula incorporated into the Bohr model.<ref name=Pais2 /> After a presentation of these results by Franck a few years later, [[Albert Einstein]] is said to have remarked, "It's so lovely it makes you cry."<ref name=Rice />

On December 10, 1926, Franck and Hertz were awarded the 1925 [[List of Nobel laureates in Physics#Laureates|Nobel Prize in Physics]] "for their discovery of the laws governing the impact of an electron upon an atom".<ref name=Oseen>{{cite web |last=Oseen |first=C. W. |author-link=Carl Wilhelm Oseen |title=Nobel Prize in Physics 1925 – Presentation Speech |date=December 10, 1926 |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1925/press.html |publisher=The Nobel Foundation}}</ref>