# Neutron number > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Neutron_number > Markdown URL: https://mediated.wiki/source/Neutron_number.md > Source: https://en.wikipedia.org/wiki/Neutron_number > Source revision: 1327091379 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{Short description|Number of neutrons in a nuclide}} {{More citations needed|date=August 2025}} [[File:Isotopes and half-life.svg|right|300px|thumb|This diagram shows the [half-life](/source/half-life) (T½) of various isotopes with Z protons and neutron number N.]] The '''neutron number''' (symbol '''''N''''') is the number of [neutron](/source/neutron)s in a [nuclide](/source/nuclide). [Atomic number](/source/Atomic_number) (proton number) plus neutron number equals [mass number](/source/mass_number): {{math|1=''Z'' + ''N'' = ''A''}}. The difference between the neutron number and the atomic number is known as the neutron excess: {{math|1=''D'' = ''N'' − ''Z'' = ''A'' − 2''Z''}}. Neutron number is not written explicitly in nuclide symbol notation, but can be inferred as it is the difference between the two left-hand numbers (atomic number and mass). {| class="wikitable" border = 0 |- style="height:2em;" | Element || C: [Carbon](/source/Carbon), no specific isotope |- style="height:2em;" | Isotope/Nuclide ||{{SimpleNuclide|Carbon|14}}: [Carbon-14](/source/Carbon-14) specifically. |- style="height:2em;" | With atomic number || {{nuclide|Carbon|14}}: Carbon-14. No more specific (carbon always has six protons) but may be more clear. |} Nuclides that have the same neutron number but different proton numbers are called [isotone](/source/isotone)s. This word was formed by replacing the '''p''' in [isotope](/source/isotope) with '''n''' for neutron. Nuclides that have the same mass number are called [isobar](/source/isobar_(nuclide))s. Nuclides that have the same neutron excess are called [isodiapher](/source/isodiapher)s.[https://books.google.com/books?id=iS1TMF_gWoYC&dq=isodiasphere&pg=PA265 Teh Fu Yen, ''Chemistry for Engineers'' (Imperial College Press, 2008), p.265] Chemical properties are primarily determined by proton number, which determines which [chemical element](/source/chemical_element) the nuclide is a member of; neutron number has only [a slight influence](/source/isotopomer). Neutron number is primarily of interest for nuclear properties. For example, [actinide](/source/actinide)s with odd neutron number are usually [fissile](/source/fissile) ([fissionable](/source/fissionable) with [slow neutron](/source/slow_neutron)s) while actinides with even neutron number are usually not fissile (but are fissionable with [fast neutron](/source/fast_neutron)s). Only 58 stable nuclides have an odd neutron number, compared to 194 with an even neutron number. No odd-neutron-number isotope is the [most naturally abundant isotope](/source/List_of_elements_by_stability_of_isotopes) in its element, except for beryllium-9 (which is the only stable [beryllium](/source/beryllium) isotope), [nitrogen-14](/source/nitrogen-14), and [platinum](/source/platinum)-195. No stable nuclides have a neutron number of 19, 21, 35, 39, 45, 61, 89, 115, 123, or ≥ 127. There are 6 stable nuclides and one radioactive [primordial nuclide](/source/primordial_nuclide) with neutron number 82 (82 is the neutron number with the most stable nuclides, since it is a [magic number](/source/magic_number_(physics))): [barium-138](/source/barium-138), [lanthanum-139](/source/lanthanum-139), [cerium-140](/source/cerium-140), [praseodymium-141](/source/praseodymium-141), [neodymium-142](/source/neodymium-142), and [samarium-144](/source/samarium-144), as well as the radioactive primordial nuclide [xenon-136](/source/xenon-136), which decays by a very slow [double beta](/source/double_beta_decay) process. Except 20, 50 and 82 (all these three numbers are magic numbers), all other neutron numbers have at most 4 stable nuclides (in the case of 20, there are 5 stable nuclides 36S, 37Cl, 38Ar, 39K, and 40Ca, and in the case for 50, there are 5 stable nuclides: 86Kr, 88Sr, 89Y, 90Zr, and 92Mo, and 1 radioactive primordial nuclide, 87Rb). Most odd neutron numbers have at most one stable nuclide (exceptions are 1 (2H and 3He), 5 (9Be and 10B), 7 (13C and 14N), 55 (97Mo and 99Ru) and 107 (179Hf and 180mTa)). However, some even neutron numbers also have only one stable nuclide; these numbers are 0 (1H), 2 (4He), 4 (7Li), 84 (142Ce), 86 (146Nd) and 126 (208Pb). The cases of 84 and 86 are special, since 142Ce and 146Nd are theoretically unstable to [double beta decay](/source/double_beta_decay), and the nuclides with 84 or 86 neutrons which are theoretically stable to both beta decay and double beta decay are 144Nd, 146Sm, and 148Sm, but all three nuclides are observed to [alpha decay](/source/alpha_decay).{{NUBASE2016|ref}} (In theory, no stable nuclides have neutron number 19, 21, 35, 39, 45, 61, 71, 83–91, 95, 96, and ≥ 99) Besides, no nuclides with neutron number 19, 21, 35, 39, 45, 61, 71, 89, 115, 123, 147, ... are stable to [beta decay](/source/beta_decay) (see [Beta-decay stable isobars](/source/Beta-decay_stable_isobars)). Only two stable nuclides have fewer neutrons than protons: [hydrogen-1](/source/hydrogen-1) and [helium-3](/source/helium-3). Hydrogen-1 has the smallest neutron number, 0. ==References== Category:Nuclear physics --- Adapted from the Wikipedia article [Neutron number](https://en.wikipedia.org/wiki/Neutron_number) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Neutron_number?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.