Editing Nucleon (section)

==Overview==
{{Main article|Proton|Neutron}}

===Properties===
{{multiple image
   | align     = right
   | direction = vertical
   | header    = Quark composition of a nucleon
   | width1    = 183
   | image1    = Quark structure proton.svg
   | alt1      = Proton
   | caption1  = Proton ({{SubatomicParticle|proton}}): {{SubatomicParticle|Up quark}}{{SubatomicParticle|Up quark}}{{SubatomicParticle|Down quark}}
   | width2    = 183
   | image2    = Quark structure neutron.svg
   | alt2      = Neutron
   | caption2  = Neutron ({{SubatomicParticle|neutron}}): {{SubatomicParticle|Up quark}}{{SubatomicParticle|Down quark}}{{SubatomicParticle|Down quark}}
   | width3    = 183
   | image3    = Quark structure antiproton.svg
   | alt3      = Antiproton
   | caption3  = Antiproton ({{SubatomicParticle|antiproton}}): {{SubatomicParticle|Up antiquark}}{{SubatomicParticle|Up antiquark}}{{SubatomicParticle|Down antiquark}}
   | width4    = 183
   | image4    = Quark structure antineutron.svg
   | alt4      = Antineutron
   | caption4  = Antineutron ({{SubatomicParticle|antineutron}}): {{SubatomicParticle|Up antiquark}}{{SubatomicParticle|Down antiquark}}{{SubatomicParticle|Down antiquark}}
   | footer    = A proton (p) is composed of two up quarks (u) and one down quark (d): uud. A neutron (n) has one up quark (u) and two down quarks (d): udd. An [[antiproton]] ({{SubatomicParticle|antiproton}}) has two up [[antiquarks]] ({{SubatomicParticle|Up antiquark}}) and one down antiquark ({{SubatomicParticle|Down antiquark}}): {{SubatomicParticle|Up antiquark}}{{SubatomicParticle|Up antiquark}}{{SubatomicParticle|Down antiquark}}. An [[antineutron]] ({{SubatomicParticle|antineutron}}) has one up antiquark ({{SubatomicParticle|Up antiquark}}) and two down antiquarks ({{SubatomicParticle|Down antiquark}}): {{SubatomicParticle|Up antiquark}}{{SubatomicParticle|Down antiquark}}{{SubatomicParticle|Down antiquark}}. The [[color charge]] ([[color charge|color assignment]]) of individual quarks is arbitrary, but all three colors (red, green, blue) must be present.
}}
Protons and neutrons are best known in their role as nucleons, i.e., as the components of atomic nuclei, but they also exist as free particles. Free neutrons are unstable, with a half-life of around 13&nbsp;minutes, but they have important applications (see [[neutron radiation]] and [[neutron scattering]]). Protons not bound to other nucleons are the nuclei of hydrogen atoms when bound with an [[electron]] or{{snd}} if not bound to anything{{snd}} are [[ion]]s or cosmic rays.<!-- particles in beams in a collider are frequently referred to as “ions”! -->

Both the proton and the neutron are [[composite particle]]s, meaning that each is composed of smaller parts, namely three [[quarks]] each; although once thought to be so, neither is an [[elementary particle]]. A proton is composed of two [[up quark]]s and one [[down quark]], while the neutron has one up quark and two down quarks. Quarks are held together by the [[strong interaction|strong force]], or equivalently, by [[gluon]]s, which mediate the strong force at the quark level.

An up quark has [[electric charge]] {{sfrac|+|2|3}}&nbsp;[[elementary charge|''e'']], and a down quark has charge {{sfrac|&minus;|1|3}}&nbsp;''e'', so the summed electric charges of proton and neutron are +[[elementary charge|''e'']] and 0, respectively.{{efn|name=coeffs|The resultant coefficients are obtained by summation of the component charges: {{math|1=<big>Σ</big>''Q'' = {{sfrac|2|3}} + {{sfrac|2|3}} + <big>(</big>{{sfrac|&minus;|1|3}}<big>)</big> = {{sfrac|3|3}} = +1}} for proton, and {{math|1=<big>Σ</big>''Q'' = {{sfrac|2|3}} + <big>(</big>{{sfrac|&minus;|1|3}}<big>)</big> + <big>(</big>{{sfrac|&minus;|1|3}}<big>)</big> = {{sfrac|0|3}} = 0}} for neutron.}} Thus, the neutron has a charge of 0 (zero), and therefore is electrically neutral; indeed, the term "neutron" comes from the fact that a neutron is electrically neutral.

The masses of the proton and neutron are similar: for the proton it is {{val|1.6726|e=-27|ul=kg}} ({{val|938.27|ul=MeV/c2}}), while for the neutron it is {{val|1.6749|e=-27|ul=kg}} ({{val|939.57|ul=MeV/c2}}); the neutron is roughly 0.13% heavier. The similarity in mass can be explained roughly by the slight difference in masses of up quarks and down quarks composing the nucleons. However, a detailed description remains an unsolved problem in particle physics.<ref name=Griffiths2008/>{{rp|135–136}}

The [[Spin (physics)|spin]] of the nucleon is {{sfrac|1|2}}, which means that they are [[fermion]]s and, like [[electron]]s, are subject to the [[Pauli exclusion principle]]: no more than one nucleon, e.g. in an atomic nucleus, may occupy the same [[quantum state]].

The [[isospin]] and [[spin quantum number|spin]] quantum numbers of the nucleon have two states each, resulting in four combinations in total. An [[alpha particle]] is composed of four nucleons occupying all four combinations, namely, it has two protons (having [[singlet state|opposite spin]]) and two neutrons (also having opposite spin), and its net [[nuclear spin]] is zero. In larger nuclei constituent nucleons, by Pauli exclusion, are compelled to have relative [[motion]], which may also contribute to nuclear spin via the [[orbital quantum number]]. They spread out into [[nuclear shell]]s analogous to [[electron shell]]s known from chemistry.

Both the proton and neutron have [[magnetic moment]]s, though the [[nucleon magnetic moment]]s are anomalous and were unexpected when they were discovered in the 1930s.  The proton's magnetic moment, symbol ''&mu;''{{sub|p}}, is {{val|2.79|u=''μ''{{sub|N}}}}, whereas, if the proton were an elementary [[Dirac particle]], it should have a magnetic moment of {{val|1.0|u=''μ''{{sub|N}}}}.  Here the unit for the magnetic moments is the [[nuclear magneton]], symbol ''μ''{{sub|N}}, an atomic-scale [[unit of measure]]. The neutron's magnetic moment is ''&mu;''{{sub|n}} = {{val|-1.91|u=''μ''{{sub|N}}}}, whereas, since the neutron lacks an electric charge, it should have no magnetic moment. The value of the neutron's magnetic moment is negative because the direction of the moment is opposite to the neutron's spin.  The nucleon magnetic moments arise from the quark substructure of the nucleons.<ref name="Perk">{{cite book |author1-last = Perkins |author1-first = Donald H. |title = Introduction to High Energy Physics |url  = https://archive.org/details/introductiontohi0000perk |url-access = registration |pages=[https://archive.org/details/introductiontohi0000perk/page/201 201–202] |publisher = Addison Wesley |location=Reading, Massachusetts |date = 1982 |isbn = 978-0-201-05757-7}}</ref><ref name="MagMom">{{cite web |url=http://phys.org/news/2015-02-magnetic-moments-nuclear.html |title=Pinpointing the magnetic moments of nuclear matter |last1=Kincade |first1=Kathy |date=2 February 2015 |publisher=Phys.org |access-date=May 8, 2015 |archive-date=2 May 2015 |archive-url=https://web.archive.org/web/20150502123656/http://phys.org/news/2015-02-magnetic-moments-nuclear.html |url-status=live }}</ref>  The proton magnetic moment is exploited for [[nuclear magnetic resonance|NMR / MRI]] scanning.

===Stability===
A neutron in free state is an unstable particle, with a [[half-life]] around ten minutes. It undergoes [[beta decay|{{SubatomicParticle|Beta-}} decay]] (a type of [[radioactive decay]]) by turning into a proton while emitting an electron and an [[electron antineutrino]]. This reaction can occur because the mass of the neutron is slightly greater than that of the proton. (See the [[Neutron]] article for more discussion of neutron decay.) A proton by itself is thought to be stable, or at least its lifetime is too long to measure. This is an important discussion in particle physics (see ''[[Proton decay]]'').

Inside a nucleus, on the other hand, combined protons and neutrons (nucleons) can be stable or unstable depending on the [[nuclide]], or nuclear species. Inside some nuclides, a neutron can turn into a proton (producing other particles) as described above; the reverse can happen inside other nuclides, where a proton turns into a neutron (producing other particles) through [[beta decay|{{SubatomicParticle|Beta+}} decay]] or [[electron capture]]. And inside still other nuclides, both protons and neutrons are stable and do not change form.

===Antinucleons===
{{Main article|Antineutron|Antiproton|Antimatter}}

Both nucleons have corresponding [[antiparticle]]s: the [[antiproton]] and the [[antineutron]], which have the same mass and opposite charge as the proton and neutron respectively, and they interact in the same way. (This is generally believed to be ''exactly'' true, due to [[CPT symmetry]]. If there is a difference, it is too small to measure in all experiments to date.) In particular, antinucleons can bind into an "antinucleus". So far, scientists have created [[antideuterium]]<ref>{{cite journal|author = Massam, T|year = 1965 |title = Experimental observation of antideuteron production |journal = Il Nuovo Cimento |volume = 39|issue = 1 |pages = 10–14 |doi = 10.1007/BF02814251|last2 = Muller|first2 = Th.|last3 = Righini|first3 = B.|last4 = Schneegans|first4 = M.|last5 = Zichichi|first5 = A.|bibcode = 1965NCimS..39...10M |s2cid = 122952224 }}</ref><ref>{{cite journal|author = Dorfan, D. E|date=June 1965|title = Observation of Antideuterons|journal = Phys. Rev. Lett.|volume = 14|issue = 24 |pages = 1003–1006| doi = 10.1103/PhysRevLett.14.1003|last2 = Eades|first2 = J.|last3 = Lederman|first3 = L. M.|last4 = Lee|first4 = W.|last5 = Ting|first5 = C. C.|bibcode=1965PhRvL..14.1003D}}</ref> and antihelium-3<ref>
{{cite journal
 |author=R. Arsenescu|year=2003
 |title=Antihelium-3 production in lead-lead collisions at 158 ''A'' GeV/''c''
 |journal=[[New Journal of Physics]]
 |volume=5 |issue=1
 |page=1
 |doi=10.1088/1367-2630/5/1/301
|bibcode = 2003NJPh....5....1A |display-authors=etal|doi-access=free}}</ref> nuclei.