The discovery that caused a great leap in quantum mechanics — Franck Hertz Experiment

Intishar Alam Misbahul
4 min readDec 6, 2020


Electron Shells

An electron is a sub-atomic particle which has properties of both of a wave and a particle and has a negative charge with almost no mass.

In 1914, German-born physicists James Franck and Gustav Hertz performed an experiment which gave evidence of discrete energy states in atoms or quantum levels of electron. A quantum level is an electron shell where each shell is represented as 1n,2n,3n….(n)n.

The experiment involved collision between low-energy electrons and mercury atoms as a vapor in a vacuum tube.

In order to do the experiment, an electron beam must be produced. This can be achieved by utilizing a filament which releases free electrons when subjected to heat through passing an electric current.

Two meshes are used as electrodes. The cathode or positive electrode is placed in front of the filament to accelerate the electrons.

A collector or a collection plate is placed after the anode or negative electrode. The anode decelerates the electrons to make sure the voltage reaching the collector is Uga(see diagram below). The filament, electrodes and the collector are placed in a vacuum tube containing mercury vapor.

A chosen potential difference or volts is set for the filament and the cathode, Ucs, and for the anode and the collector, Uga. Ucs and Uga will be the same and fixed. U will be a voltage that is increased to a specified limit.

The graph represents a similar result found by Franck and Hertz. If you look at the graph, when ever there was a voltage increase by 4.9, there is a sudden drop in current(rate of flow of charge). What might have caused this? What reduced the current so dramatically? The electron beam, when reached a certain voltage are able to ‘excite’ the electrons of mercury, making the electron jump from a lower state to a higher state, then lose the energy to go back down. After exciting the mercury electron, the electron that collided completely lose their momentum and not reach the collector. The 4.9 volts of electron kinetic energy corresponded to a wavelength of 253,6nm. The wavelength is that of UV ray. When a UV detector was placed near the tube, readings were recorded every time there was a drop in current. The electron loses their energy in form of light or EM waves.

This result gave evidence for explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. Which proved the concept of discrete energy states of electrons.

The year before the experiment, Neils Bohr proposed that electrons do not revolve around the atom like a miniature version of the solar system with arbitrary radii, but instead with discrete radii.

This model gave birth to electron shells.

Extra: Electron shells are also called quantum shells. Higher the shell number means higher the quantum level, thus higher the energy level. As electrons with more energy take place in higher shells while lower energy ones closer to the nucleus.


Electron Subshells and Orbitals

The Bohr model is useful to explain the reactivity and chemical bonding of many elements, but when comes to explaining how they are distributed…it is not really helpful. Specifically, electrons don’t really circle the nucleus, but rather spend most of their time in sometimes-complex-shaped regions of space around the nucleus, known as electron orbitals. We can’t actually know where an electron is at any given moment in time, but we can mathematically determine the volume of space in which it is most likely to be found — say, the volume of space in which it will spend 90% of its time. This high-probability region makes up an orbital, and each orbital can hold up to two electrons.

They are sometimes called electron clouds.

Now how do these mathematically defined orbitals fit in with the electron shells we saw in the Bohr model? We can break each electron shell down into one or more subshells, which are simply sets of one or more orbitals. Subshells are designated by the letters s, p, d, and f, and each letter indicates a different shape.

s-orbital shape
p-orbitals shapes
d-orbitals shapes
f-orbitals shapes

In a nutshell(subshells)

Mathematically defined shapes of electron distribution



Intishar Alam Misbahul

Currently 17. Loves to learn. Doing AS level. Using medium to write easy-to-understand essays/stories regarding science and study tips.