Editing Atom (section)

=== Electron cloud ===
{{Main|Electron configuration|Electron shell|Atomic orbital}}{{See also|Electronegativity}}[[File:Potential energy well.svg|right|thumb|A potential well, showing, according to [[classical mechanics]], the minimum energy ''V''(''x'') needed to reach each position ''x''. Classically, a particle with energy ''E'' is constrained to a range of positions between ''x''<sub>1</sub> and ''x''<sub>2</sub>.]]
The electrons in an atom are attracted to the protons in the nucleus by the [[electromagnetic force]]. This force binds the electrons inside an [[electrostatic]] [[potential well]] surrounding the smaller nucleus, which means that an external source of energy is needed for the electron to escape. The closer an electron is to the nucleus, the greater the attractive force. Hence electrons bound near the center of the potential well require more energy to escape than those at greater separations.

Electrons, like other particles, have properties of both a [[wave–particle duality|particle and a wave]]. The electron cloud is a region inside the potential well where each electron forms a type of three-dimensional [[standing wave]]—a wave form that does not move relative to the nucleus. This behavior is defined by an [[atomic orbital]], a mathematical function that characterises the probability that an electron appears to be at a particular location when its position is measured.<ref name=science157_3784_13 /> Only a discrete (or [[wikt:quantize|quantized]]) set of these orbitals exist around the nucleus, as other possible wave patterns rapidly decay into a more stable form.<ref name=Brucat2008 /> Orbitals can have one or more ring or node structures, and differ from each other in size, shape and orientation.<ref name=manthey2001 />

[[File:Atomic-orbital-clouds spdf m0.png|thumb|upright=1.5|3D views of some [[Hydrogen-like atom|hydrogen-like]] atomic orbitals showing probability density and phase ('''g''' orbitals and higher are not shown)]]

Each atomic orbital corresponds to a particular [[energy level]] of the electron. The electron can change its state to a higher energy level by absorbing a [[photon]] with sufficient energy to boost it into the new quantum state. Likewise, through [[spontaneous emission]], an electron in a higher energy state can drop to a lower energy state while radiating the excess energy as a photon. These characteristic energy values, defined by the differences in the energies of the quantum states, are responsible for [[atomic spectral line]]s.<ref name=Brucat2008 />

The amount of energy needed to remove or add an electron—the [[electron binding energy]]—is far less than the [[binding energy|binding energy of nucleons]]. For example, it requires only 13.6&nbsp;eV to strip a [[Stationary state|ground-state]] electron from a hydrogen atom,<ref name=herter_8 /> compared to 2.23&nbsp;''million'' eV for splitting a [[deuterium]] nucleus.<ref name=pr79_2_282 /> Atoms are [[electric charge|electrically]] neutral if they have an equal number of protons and electrons. Atoms that have either a deficit or a surplus of electrons are called [[ion]]s. Electrons that are farthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism, atoms are able to [[chemical bond|bond]] into [[molecule]]s and other types of [[chemical compound]]s like [[Ionic crystal|ionic]] and [[Covalent bond|covalent]] network [[Crystallization|crystals]].<ref>{{cite book|last=Smirnov|first=Boris M.|year=2003|title=Physics of Atoms and Ions|url=https://archive.org/details/physicsatomsions00smir|url-access=limited|publisher=Springer|isbn=978-0-387-95550-6|pages=[https://archive.org/details/physicsatomsions00smir/page/n262 249]–272}}</ref>