Editing Atom (section)

== Structure ==
=== Subatomic particles ===
{{Main|Subatomic particle}}
Though the word ''atom'' originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various [[subatomic particle]]s. The constituent particles of an atom are the [[electron]], the [[proton]], and the [[neutron]].

The electron is the least massive of these particles by four orders of magnitude at {{val|9.11|e=-31|u=kg}}, with a negative [[Electric charge|electrical charge]] and a size that is too small to be measured using available techniques.<ref>{{cite book|last=Demtröder|first=Wolfgang|year=2002|title=Atoms, Molecules and Photons: An Introduction to Atomic- Molecular- and Quantum Physics|url=https://archive.org/details/atomsmoleculesph00demt_277|url-access=limited|publisher=Springer|edition=1st|isbn=978-3-540-20631-6|oclc=181435713|pages=[https://archive.org/details/atomsmoleculesph00demt_277/page/n51 39]–42}}</ref> It was the lightest particle with a positive rest mass measured, until the discovery of [[neutrino]] mass. Under ordinary conditions, electrons are bound to the positively charged nucleus by the attraction created from opposite electric charges. If an atom has more or fewer electrons than its atomic number, then it becomes respectively negatively or positively charged as a whole; a charged atom is called an [[ion]]. Electrons have been known since the late 19th century, mostly thanks to [[J.J. Thomson]]; see [[history of subatomic physics]] for details.

Protons have a positive charge and a mass of {{val|1.6726|e=-27|u=kg}}. The number of protons in an atom is called its [[atomic number]]. [[Ernest Rutherford]] (1919) observed that nitrogen under alpha-particle bombardment ejects what appeared to be hydrogen nuclei. By 1920 he had accepted that the hydrogen nucleus is a distinct particle within the atom and named it [[proton]].

Neutrons have no electrical charge and have a mass of {{val|1.6749|e=-27|u=kg}}.<ref>{{cite book|last=Woan|first=Graham|year=2000|title=The Cambridge Handbook of Physics|publisher=Cambridge University Press|isbn=978-0-521-57507-2|oclc=224032426|page=[https://archive.org/details/cambridgehandboo0000woan/page/8 8]|url=https://archive.org/details/cambridgehandboo0000woan/page/8}}</ref><ref name="2014 CODATA">Mohr, P.J.; Taylor, B.N. and Newell, D.B. (2014), [http://physics.nist.gov/constants "The 2014 CODATA Recommended Values of the Fundamental Physical Constants"] {{Webarchive|url=https://web.archive.org/web/20120211083747/http://physics.nist.gov/cuu/Constants/index.html |date=11 February 2012 }} (Web Version 7.0). The database was developed by J. Baker, M. Douma, and [[Svetlana Kotochigova|S. Kotochigova]]. (2014). National Institute of Standards and Technology, Gaithersburg, Maryland 20899.</ref> Neutrons are the heaviest of the three constituent particles, but their mass can be reduced by the [[nuclear binding energy]]. Neutrons and protons (collectively known as [[nucleon]]s) have comparable dimensions—on the order of {{val|2.5|e=-15|u=m}}—although the 'surface' of these particles is not sharply defined.<ref>{{cite book|last=MacGregor|first=Malcolm H.|year=1992|title=The Enigmatic Electron|publisher=Oxford University Press|isbn=978-0-19-521833-6|oclc=223372888|pages=[https://archive.org/details/astronomyencyclo0000unse/page/33 33–37]|url=https://archive.org/details/astronomyencyclo0000unse/page/33}}</ref> The neutron was discovered in 1932 by the English physicist [[James Chadwick]].

In the [[Standard Model]] of physics, electrons are truly elementary particles with no internal structure, whereas protons and neutrons are composite particles composed of [[elementary particle]]s called [[quark]]s. There are two types of quarks in atoms, each having a fractional electric charge. Protons are composed of two [[up quark]]s (each with charge +{{sfrac|2|3}}) and one [[down quark]] (with a charge of −{{sfrac|1|3}}). Neutrons consist of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles.<ref name=pdg2002 /><ref name=schombert2006 />

The quarks are held together by the [[strong interaction]] (or strong force), which is mediated by [[gluon]]s. The protons and neutrons, in turn, are held to each other in the nucleus by the [[nuclear force]], which is a residuum of the strong force that has somewhat different range-properties (see the article on the nuclear force for more). The gluon is a member of the family of [[gauge boson]]s, which are elementary particles that mediate physical forces.<ref name=pdg2002 /><ref name=schombert2006 />

=== Nucleus ===
{{Main|Atomic nucleus}}
[[File:Binding energy curve - common isotopes.svg|thumb|The [[binding energy]] needed for a nucleon to escape the nucleus, for various isotopes]]<!-- A brief explanation is provided here because 'binding energy' is not explained until the end of the section. -->

All the bound protons and neutrons in an atom make up a tiny [[atomic nucleus]], and are collectively called [[nucleon]]s. The radius of a nucleus is approximately equal to <math>1.07 \sqrt[3]{A}</math>&nbsp;[[femtometre]]s, where <math>A</math> is the total number of nucleons.<ref>{{cite book|last=Jevremovic|first=Tatjana|year=2005|title=Nuclear Principles in Engineering|url=https://archive.org/details/nuclearprinciple00jevr_450|url-access=limited|publisher=Springer|isbn=978-0-387-23284-3|oclc=228384008|page=[https://archive.org/details/nuclearprinciple00jevr_450/page/n83 63]}}</ref> This is much smaller than the radius of the atom, which is on the order of 10<sup>5</sup>&nbsp;fm. The nucleons are bound together by a short-ranged attractive potential called the [[residual strong force]]. At distances smaller than 2.5&nbsp;fm this force is much more powerful than the [[electrostatic force]] that causes positively charged protons to repel each other.<ref>{{cite book|last1=Pfeffer|first1=Jeremy I.|last2=Nir|first2=Shlomo|year=2000|title=Modern Physics: An Introductory Text|publisher=Imperial College Press|isbn=978-1-86094-250-1|oclc=45900880|pages=330–336}}</ref>

Atoms of the same [[chemical element|element]] have the same number of protons, called the [[atomic number]]. Within a single element, the number of neutrons may vary, determining the [[isotope]] of that element. The total number of protons and neutrons determine the [[nuclide]]. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing [[radioactive decay]].<ref name=wenner2007 />

The proton, the electron, and the neutron are classified as [[fermion]]s. Fermions obey the [[Pauli exclusion principle]] which prohibits ''[[identical particles|identical]]'' fermions, such as multiple protons, from occupying the same quantum state at the same time. Thus, every proton in the nucleus must occupy a quantum state different from all other protons, and the same applies to all neutrons of the nucleus and to all electrons of the electron cloud.<ref name="raymond" />

A nucleus that has a different number of protons than neutrons can potentially drop to a lower energy state through a radioactive decay that causes the number of protons and neutrons to more closely match. As a result, atoms with matching numbers of protons and neutrons are more stable against decay, but with increasing atomic number, the mutual repulsion of the protons requires an increasing proportion of neutrons to maintain the stability of the nucleus.<ref name="raymond" />

[[File:Wpdms physics proton proton chain 1.svg|right|thumb|upright|Illustration of a nuclear fusion process that forms a deuterium nucleus, consisting of a proton and a neutron, from two protons. A [[positron]] (e<sup>+</sup>)—an [[antimatter]] electron—is emitted along with an electron [[neutrino]].]]

The number of protons and neutrons in the atomic nucleus can be modified, although this can require very high energies because of the strong force. [[Nuclear fusion]] occurs when multiple atomic particles join to form a heavier nucleus, such as through the energetic collision of two nuclei. For example, at the core of the Sun protons require energies of 3 to 10&nbsp;keV to overcome their mutual repulsion—the [[coulomb barrier]]—and fuse together into a single nucleus.<ref name=mihos2002 /> [[Nuclear fission]] is the opposite process, causing a nucleus to split into two smaller nuclei—usually through radioactive decay. The nucleus can also be modified through bombardment by high energy subatomic particles or photons. If this modifies the number of protons in a nucleus, the atom changes to a different chemical element.<ref name=lbnl20070330 /><ref name=makhijani_saleska2001 />

If the mass of the nucleus following a fusion reaction is less than the sum of the masses of the separate particles, then the difference between these two values can be emitted as a type of usable energy (such as a [[gamma ray]], or the kinetic energy of a [[beta particle]]), as described by [[Albert Einstein]]'s [[mass–energy equivalence]] formula, ''E = mc<sup>2</sup>'', where ''m'' is the mass loss and ''c'' is the [[speed of light]]. This deficit is part of the [[binding energy]] of the new nucleus, and it is the non-recoverable loss of the energy that causes the fused particles to remain together in a state that requires this energy to separate.<ref>{{cite book|last1=Shultis|first1=J. Kenneth|last2=Faw|first2=Richard E.|title=Fundamentals of Nuclear Science and Engineering|year=2002|publisher=CRC Press|isbn=978-0-8247-0834-4|oclc=123346507|pages=10–17}}</ref>

The fusion of two nuclei that create larger nuclei with lower atomic numbers than [[iron]] and [[nickel]]—a total nucleon number of about 60—is usually an [[exothermic reaction|exothermic process]] that releases more energy than is required to bring them together.<ref name=ajp63_7_653 /> It is this energy-releasing process that makes nuclear fusion in [[star]]s a self-sustaining reaction.  For heavier nuclei, the binding energy per [[nucleon]] begins to decrease. That means that a fusion process producing a nucleus that has an atomic number higher than about 26, and a [[mass number]] higher than about 60, is an [[endothermic reaction|endothermic process]]. Thus, more massive nuclei cannot undergo an energy-producing fusion reaction that can sustain the [[hydrostatic equilibrium]] of a star.<ref name="raymond" />

=== 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>