Editing Neutron (section)

=== Atomic nucleus ===
{{Main|Atomic nucleus|Nuclear physics}}
{{See also|Valley of stability|Beta-decay stable isobars|Neutron emission}}
{{Nuclear physics}}
An atomic nucleus is formed by a number of protons, ''Z'' (the [[atomic number]]), and a number of neutrons, ''N'' (the [[neutron number]]), bound together by the [[nuclear force]]. Protons and neutrons each have a mass of approximately one [[dalton (unit)|dalton]]. The atomic number determines the [[chemical element|chemical properties]] of the atom, and the neutron number determines the [[isotope]] or [[nuclide]].<ref name="ENW">{{Citation |editor1-last= Glasstone |editor1-first= Samuel |editor2-last= Dolan |editor2-first= Philip J. |title= The Effects of Nuclear Weapons |edition=3rd |publisher= U.S. Dept. of Defense and Energy Research and Development Administration, U.S. Government Printing Office |date= 1977 |isbn= 978-1-60322-016-3}}</ref>{{rp|4}} The terms isotope and nuclide are often used [[synonym]]ously, but they refer to chemical and nuclear properties, respectively.<ref name="ENW"/>{{rp|4}} Isotopes are nuclides with the same atomic number, but different neutron number. Nuclides with the same neutron number, but different atomic number, are called [[isotone]]s.<ref name="Brucer">{{Cite journal| last=Brucer| first=Marshall| year=1978| title=Nuclear Medicine Begins with a Boa Constrictor| journal=J. Nuclear Medicine| volume=19| issue=6| pages=581–598| pmid=351151| url=http://jnm.snmjournals.org/content/19/6/581.full.pdf| access-date=2024-05-01| archive-date=2019-05-09| archive-url=https://web.archive.org/web/20190509183540/http://jnm.snmjournals.org/content/19/6/581.full.pdf| url-status=live}}</ref> The [[atomic mass number]], ''A'', is equal to the sum of atomic and neutron numbers. Nuclides with the same atomic mass number, but different atomic and neutron numbers, are called [[isobar (nuclide)|isobars]].<ref name=Brucer/>  The mass of a nucleus is always slightly less than the sum of its proton and neutron masses: the difference in mass represents the [[Mass–energy equivalence|mass equivalent]] to nuclear binding energy, the energy which would need to be added to take the nucleus apart.<ref>{{Cite book |last=Giancoli |first=Douglas C. |url=https://archive.org/details/generalphysics00gian |title=General physics |date=1984 |publisher=Prentice-Hall |isbn=978-0-13-350884-0 |location=Englewood Cliffs, N.J |oclc=1033640549}}</ref>{{rp|822}}

The nucleus of the most common [[isotope]] of the [[hydrogen atom]] (with the [[chemical symbol]] <sup>1</sup>H) is a lone proton.<ref name="ENW"/>{{rp|20}} The nuclei of the heavy hydrogen isotopes [[deuterium]] (D or <sup>2</sup>H) and [[tritium]] (T or <sup>3</sup>H) contain one proton bound to one and two neutrons, respectively.<ref name="ENW"/>{{rp|20}} All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons. The most common nuclide of the common chemical element [[lead]], <sup>208</sup>Pb, has 82 protons and 126 neutrons, for example.<ref name="Stone">{{cite journal |last=Stone |first=R. |year=1997 |title=An Element of Stability |journal=[[Science (journal)|Science]] |volume=278 |issue=5338 |pages=571–572 |doi=10.1126/science.278.5338.571|bibcode=1997Sci...278..571S |s2cid=117946028 }}</ref> The [[table of nuclides]] comprises all the known nuclides. Even though it is not a chemical element, the neutron is included in this table.<ref>[http://www.nndc.bnl.gov/nudat2 Nudat 2] {{Webarchive|url=https://web.archive.org/web/20090817135107/http://www1.nndc.bnl.gov/nudat2/ |date=2009-08-17 }}. Nndc.bnl.gov. Retrieved on 2010-12-04.</ref>

[[File:Nuclear fission.svg|thumb|right|Nuclear fission caused by absorption of a neutron by uranium-235. The heavy nuclide fragments into lighter components and additional neutrons.]]

Protons and neutrons behave almost identically under the influence of the nuclear force within the nucleus. They are therefore both referred to collectively as [[nucleon]]s.<ref name="Nuc">{{Citation |author1-last= Thomas |author1-first= A.W. |author2-last= Weise |author2-first= W. |title= The Structure of the Nucleon |publisher= Wiley-WCH, Berlin |date= 2001 |isbn= 978-3-527-40297-7}}</ref>  The concept of [[isospin]], in which the proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces.<ref name=Greiner>{{cite book|last1=Greiner|first1=W.|last2=Müller|first2=B.|author-link1=Walter Greiner|title=Quantum Mechanics: Symmetries|year=1994|edition=2nd|isbn=978-3540580805|publisher=Springer|url=https://archive.org/details/quantummechanics0001grei|url-access=registration|page=[https://archive.org/details/quantummechanics0001grei/page/279 279]}}</ref>{{rp|141}}

Neutrons are a necessary constituent of any atomic nucleus that contains more than one proton. As a result of their positive charges, interacting protons have a mutual [[electromagnetic interaction|electromagnetic repulsion]] that is stronger than their attractive [[nuclear force|nuclear interaction]], so proton-only nuclei are unstable (see [[diproton]] and [[neutron–proton ratio]]).<ref>[http://ansnuclearcafe.org/2011/10/19/pioneers102011/ Sir James Chadwick's Discovery of Neutrons] {{Webarchive|url=https://web.archive.org/web/20111026055138/http://ansnuclearcafe.org/2011/10/19/pioneers102011/ |date=2011-10-26 }}. ANS Nuclear Cafe. Retrieved on 2012-08-16.</ref> Neutrons bind with protons and one another in the nucleus via the [[nuclear force]], effectively moderating the repulsive forces between the protons and stabilizing the nucleus.<ref name="Pais1993"/>{{rp|461}} Heavy nuclei carry a large positive charge, hence they require "extra" neutrons to be stable.<ref name="Pais1993"/>{{rp|461}}

While a free neutron is unstable and a free proton is stable, within nuclei neutrons are often stable and protons are sometimes unstable. When bound within a nucleus, nucleons can [[radioactive decay|decay]] by the beta decay process. The neutrons and protons in a nucleus form a [[introduction to quantum mechanics|quantum mechanical system]] according to the [[nuclear shell model]]. Protons and neutrons of a [[nuclide]] are organized into discrete hierarchical [[energy level]]s with unique [[quantum numbers]]. Nucleon decay within a nucleus can occur if allowed by basic energy conservation and quantum mechanical constraints. The decay products, that is, the emitted particles, carry away the energy excess as a nucleon falls from one quantum state to one with less energy, while the neutron (or proton) changes to a proton (or neutron).

For a neutron to decay, the resulting proton requires an available state at lower energy than the initial neutron state. In stable nuclei the possible lower energy states are all filled, meaning each state is occupied by a pair of protons, one with [[Spin (physics)|spin]] up, another with spin down. When all available proton states are filled, the [[Pauli exclusion principle]] disallows the decay of a neutron to a proton.<ref name="Byrne_NNM">Byrne, J. ''Neutrons, Nuclei, and Matter'', Dover Publications, Mineola, New York, 2011, {{ISBN|0486482383}}</ref>{{rp|§3.3}} The situation is similar to electrons of an atom, where electrons that occupy distinct [[atomic orbital]]s are prevented by the exclusion principle from decaying to lower, already-occupied, energy states.<ref name="Byrne_NNM"/>{{rp|§3.3}} The [[stability of matter]] is a consequence of these constraints.<ref name="DysonI">{{Cite journal|last1=Dyson| first1=F. J. |last2=Lenard| first2=A. |title= Stability of Matter. I|journal= Journal of Mathematical Physics|volume=8|pages=423–434|year=1967| issue=3 | doi=10.1063/1.1705209 | bibcode=1967JMP.....8..423D }}</ref><ref name="DysonII">{{Cite journal|last1=Dyson| first1=F. J. |last2=Lenard| first2=A. |title= Stability of Matter. II|journal= Journal of Mathematical Physics|volume=9|pages=698–711|year=1968| issue=5 | doi=10.1063/1.1664631 | bibcode=1968JMP.....9..698L }}</ref><ref name="Ball">{{Cite journal | last=Ball | first=Philip | date=17 February 2021 | title=Why is matter stable? | journal=Chemistry World | url=https://www.chemistryworld.com/opinion/why-is-matter-stable/4013146.article | access-date=8 May 2024 | archive-date=8 May 2024 | archive-url=https://web.archive.org/web/20240508191013/https://www.chemistryworld.com/opinion/why-is-matter-stable/4013146.article | url-status=live }}</ref>

The decay of a neutron within a nuclide is illustrated by the decay of the [[carbon]] isotope [[carbon-14]], which has 6 protons and 8 neutrons. With its excess of neutrons, this isotope decays by beta decay to [[nitrogen-14]] (7 protons, 7 neutrons), a process with a half-life of about {{val|5730|u=years|fmt=commas}}.<ref name="McKie">{{Cite journal| last=McKie | first=Robin | date=10 August 2019 | title= 'Perhaps the most important isotope': how carbon-14 revolutionised science| journal=The Guardian|url=https://www.theguardian.com/science/2019/aug/10/most-important-isotope-how-carbon-14-revolutionised-science|access-date=8 May 2024}}</ref> Nitrogen-14 is stable.<ref name="Nova">{{Cite journal | date=10 August 2019 | title= Close Encounters (of the Cosmic Kind)| journal=PBS: Nova Online|url=https://www.pbs.org/wgbh/nova/first/radiocarbonce.html|access-date=8 May 2024}}</ref>