Electron cloud

Electron cloud is a term used, if not originally coined, by the Nobel Prize laureate and acclaimed educator Richard Feynman in The Feynman Lectures on Physics (Feynman2006 Vol 1 lect 6 pg 11) for discussing "exactly what is an electron?". This intuitive model provides a simplified way of visualizing an electron as a solution of the Schrödinger equation, an advancement using the scientific method to surprising observations that could only be explained by introducing randomness. In the electron cloud analogy, the probability density of an electron, or wavefunction, is described as a small cloud moving around the atomic or molecular nucleus, with the opacity of the cloud proportional to the probability density.

The model evolved from the earlier Bohr model, which likened an electron orbiting an atomic nucleus to a planet orbiting the sun. The electron cloud formulation better describes many observed phenomena, including the double slit experiment, the periodic table and chemical bonding, and atomic interactions with light. Although lacking in certain details, the intuitive model roughly predicts the experimentally observed wave-particle duality, in that electron behavior is described as a delocalized wavelike object, yet compact enough to be considered a particle on certain length-scales.

Experimental evidence suggests that the probability density is not just a theoretical model for the uncertainty in the location of the electron, but rather that it reflects the actual state of the electron. This carries an enormous philosophical implication, indicating that point-like particles do not actually exist, and that the universe's evolution may be fundamentally uncertain. The fundamental source of quantum uncertainty is an unsolved problem in physics.

In the electron cloud model, rather than following fixed orbits, electrons bound to an atom are observed more frequently in certain areas around the nucleus called orbitals. The electron cloud can transition between electron orbital states, and each state has a characteristic shape and energy, all predicted by the Schrödinger equation, which has infinitely many solutions. Experimental results motivated this conceptual refinement of the Bohr model. The famous double slit experiment demonstrates the random behavior of electrons, as free electrons shot through a double slit are observed at random locations at a screen, consistent with wavelike interference. Heisenberg's uncertainty principle accounts for this and, taken together with the double slit experiment, implies that an electron behaves like a spread of infinitesimal pieces, or "cloud", each piece moving somewhat independently as in a churning cloud. These pieces can be forced to coincide at an isolated point in time, but then they all must move relative to each other at an increased spread of rates to conserve the "uncertainty". Certain physical interactions of this wavelike electron, such as observing which slit an electron passes through in the double-slit experiment, require this coincidence of pieces into a lump-like particle. In such an interaction the electron "materializes", "lumps", or "is observed" at the location of one of the infinitesimal pieces, apparently randomly chosen. Although the cloud shrinks to the accuracy of the observation (if observed by light for example the wavelength of the light limits the accuracy), its momentum spread increases so that Heisenberg's uncertainty principle is still valid.

References

* Feynman, Richard; Leighton; Sands. (2006). "The Feynman Lectures on Physics -The Definitive Edition- ". Pearson Addison Wesley. ISBN 0-8053-9046-4