Isotone

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). 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.