:This article is about the structure of an atom. For the particle accelerator phenomenon, see Electron-Cloud Effect.
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 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. 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.
Unlike the fixed orbit conceptualization, an electron cloud bound in an atom is not predicted to collapse towards the charge nucleus, while emitting photons, in order to minimize the sum of electric potential and kinetic energies, since the "cloud" would gain too much kinetic energy, as required to conserve uncertainty. The smear obeys Schrödinger's equation (see also Erwin Schrödinger), which has discrete solutions at differing energy levels. These solutions are often depicted with density scatter plots or grayscale maps, which resemble a cloud. This predicts light interactions with an atom, as electrons transition between these cloud states by absorbing or emitting photons equivalent to the difference, or quantum, in their energy. Also, the periodic table is predicted as an electron is added to the lowest unoccupied energy orbital in progressing from hydrogen to helium, and to subsequent elements, with properties that match those predicted by the orbital solutions to Schrödinger's equation.
The term "electron cloud" carries specific connotations in atomic and particle physics, where everyday experience does not extrapolate well. Additional experiments, such as the behavior of electrons in high speed accelerators, have resulted in more sophisticated models including quantum electrodynamics and quantum field theory. However, what drives the uncertainty in the electron cloud model remains one of the great unsolved mysteries of physics.
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