The largest numbers of observationally stable nuclides exist for isotones 50 (five: Kr, Sr, Y, Zr, Mo – noting also the primordial radionuclide Rb) and 82 (six: Ba, La, Ce, Pr, Nd, Sm – noting also the primordial radionuclide Xe). 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 (H and He), 5 (Be and B), 7 (C and N), 55 (Mo and Ru), and 107 (Hf and Ta). 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 (V), 65 (Cd), 81 (La), 84 (Nd), 85 (Sm), 86 (Sm), 105 (Lu), and 126 (Bi). Neutron numbers for which there are two primordial radionuclides are 88 (Eu and Gd) and 112 (Re and Pt).