Innuclear physics, isotones are nucleides of the different chemical elements. They have the same neutron number, but different nucleon number (mass number) due to different proton number (atomic number). For example, boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones. Similarly, 36S, 37Cl, 38Ar, 39K, and 40Ca nuclei are all isotones of 20 because they all contain 20 neutrons.
The term derives from Greek ἴσος (isos) 'same', and τόνος (tonos) 'tension' metaphorically suggesting the uniformity in one property (neutron count). It was formed by the German physicist K. Guggenheimer[1] by changing the "p" in "isotope" from "p" for "proton" to "n" for "neutron".[2]
The largest numbers of observationally stable nuclides exist for isotones 50 (five: 86Kr, 88Sr, 89Y, 90Zr, 92Mo – noting also the primordial radionuclide 87Rb) and 82 (six: 138Ba, 139La, 140Ce, 141Pr, 142Nd, 144Sm – noting also the primordial radionuclide 136Xe). Neutron numbers for which there are no stable isotones are 19, 21, 35, 39, 45, 61, 89, 115, 123, and 127 or more (though 21, 142, 143, 146, and perhaps 150 have primordial radionuclides). In contrast, the proton numbers for which there are no stable isotopes are 43, 61, and 83 or more (83, 90, 92, and perhaps 94 have primordial radionuclides).[3] This is related to nuclear magic numbers, the number of nucleons forming complete shells within the nucleus, e.g. 2, 8, 20, 28, 50, 82, and 126. No more than one observationally stable nuclide has the same odd neutron number, except for 1 (2H and 3He), 5 (9Be and 10B), 7 (13C and 14N), 55 (97Mo and 99Ru), and 107 (179Hf and 180mTa). In contrast, all even neutron numbers from 6 to 124, except 84 and 86, have at least two observationally stable nuclides. Neutron numbers for which there is a stable nuclide and a primordial radionuclide are 27 (50V), 65 (113Cd), 81 (138La), 84 (144Nd), 85 (147Sm), 86 (148Sm), 105 (176Lu), and 126 (209Bi). Neutron numbers for which there are two primordial radionuclides are 88 (151Eu and 152Gd) and 112 (187Re and 190Pt).
The neutron numbers which have only one stable nuclide (compare: monoisotopic element for the proton numbers) are: 0, 2, 3, 4, 9, 11, 13, 15, 17, 23, 25, 27, 29, 31, 33, 37, 41, 43, 47, 49, 51, 53, 57, 59, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 84, 85, 86, 87, 91, 93, 95, 97, 99, 101, 103, 105, 109, 111, 113, 117, 119, 121, 125, 126, and the neutron numbers which have only one significant naturally-abundant nuclide (compare: mononuclidic element for the proton numbers) are: 0, 2, 3, 4, 9, 11, 13, 15, 17, 21, 23, 25, 29, 31, 33, 37, 41, 43, 47, 49, 51, 53, 57, 59, 63, 67, 69, 71, 73, 75, 77, 79, 83, 87, 91, 93, 95, 97, 99, 101, 103, 109, 111, 113, 117, 119, 121, 125, 142, 143, 146.
