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:''This article is about the structure of an atom. For the particle accelerator phenomenon, see ].'' |
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'''Electron cloud''' is a term used, if not originally coined, by the ] laureate and acclaimed educator ] in ] (] Vol 1 lect 6 pg 11) for discussing "exactly what is an ]?". This intuitive model provides a simplified way of visualizing an electron as a solution of the ], an advancement using the ] to surprising observations that could only be explained by introducing randomness. In the electron cloud analogy, the probability density of an electron, or ], is described as a small cloud moving around the ] or ], with the opacity of the cloud proportional to the probability density. |
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The model evolved from the earlier ], which likened an electron ]ing an atomic nucleus to a planet orbiting the sun. The electron cloud formulation better describes many observed phenomena, including the ], the ] and ], and atomic interactions with light. Although lacking in certain details, the intuitive model roughly predicts the experimentally observed ], in that electron behavior is described as a delocalized wavelike object, yet compact enough to be considered a particle on certain length-scales. |
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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. |
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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 ]s. 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. ] 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. |
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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 ] (see also ]), which has discrete solutions at differing energy levels. These solutions are often depicted with density ]s or ] 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. |
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The term "electron cloud" is an example of the power of simple terminology to facilitate progress and understanding. Everyday experience does not extrapolate easily to small scale physics, and Feynman's characteristic ability to "find the easy way" was central to his exceptional contributions in the advancement of physics. |
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Additional experiments, such as the behavior of electrons in high speed accelerators, have resulted in more sophisticated models including ] and ]. However, what drives the uncertainty in the electron cloud model remains one of the great unsolved mysteries of physics. |
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== See also == |
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* ] |
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== References == |
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<cite id=Feynman2006>* Feynman, Richard; Leighton; Sands. (2006). ''The Feynman Lectures on Physics -The Definitive Edition- ''. Pearson Addison Wesley. ISBN 0-8053-9046-4</cite> |
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