Neutron number
Theneutron number(symbolN) is the number ofneutronsin anuclide.
Atomic number(proton number) plus neutron number equalsmass number:Z+N=A.The difference between the neutron number and the atomic number is known as the neutron excess:D=N−Z=A− 2Z.
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).
| Element | C:Carbon,no specific isotope |
| Isotope/Nuclide | 14 C :Carbon-14specifically. |
| With atomic number | 14 6C :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 calledisotones.This word was formed by replacing thepinisotopewithnfor neutron. Nuclides that have the same mass number are calledisobars.Nuclides that have the same neutron excess are calledisodiaphers.[1]
Chemical properties are primarily determined by proton number, which determines whichchemical elementthe nuclide is a member of; neutron number has onlya slight influence.
Neutron number is primarily of interest for nuclear properties. For example,actinideswith odd neutron number are usuallyfissile(fissionablewithslow neutrons) while actinides with even neutron number are usually not fissile (but are fissionable withfast neutrons).
Only 58 stable nuclides have an odd neutron number, compared to 194 with an even neutron number. No odd-neutron-number isotope is themost naturally abundant isotopein its element, except for beryllium-9 (which is the only stableberylliumisotope),nitrogen-14,andplatinum-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 radioactiveprimordial nuclidewith neutron number 82 (82 is the neutron number with the most stable nuclides, since it is amagic number):barium-138,lanthanum-139,cerium-140,praseodymium-141,neodymium-142,andsamarium-144,as well as the radioactive primordial nuclidexenon-136,which decays by a very slowdouble betaprocess. 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 nuclides36S,37Cl,38Ar,39K, and40Ca, and in the case for 50, there are 5 stable nuclides:86Kr,88Sr,89Y,90Zr, and92Mo, and 1 radioactive primordial nuclide,87Rb). Most odd neutron numbers have at most one stable nuclide (exceptions are 1 (2H and3He), 5 (9Be and10B), 7 (13C and14N), 55 (97Mo and99Ru) and 107 (179Hf and180mTa)). 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 case of 84 is special, since142Ce is theoretically unstable todouble beta decay,and the nuclides with 84 neutrons which are theoretically stable to both beta decay and double beta decay are144Nd and146Sm, but both nuclides are observed toalpha decay.[2](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 tobeta decay(seeBeta-decay stable isobars).
Only two stable nuclides have fewer neutrons than protons:hydrogen-1andhelium-3.Hydrogen-1 has the smallest neutron number, 0.
References
[edit]- ^Teh Fu Yen,Chemistry for Engineers(Imperial College Press, 2008), p.265
- ^Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017)."The NUBASE2016 evaluation of nuclear properties"(PDF).Chinese Physics C.41(3): 030001.Bibcode:2017ChPhC..41c0001A.doi:10.1088/1674-1137/41/3/030001.