Isotone

Despite its similarity to the Greek for "same stretching", the term was formed by the German physicist K. Guggenheimer[1] by changing the "p" in "isotope" from "p" for "proton" to "n" for "neutron".

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).

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.