What is "isotone"

n. (context physics English) either of a number of nuclides of different elements that have the same number of neutrons

Two nuclides are isotones if they have the very same neutron number N, but different proton number Z. For example, boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones . Similarly, S, Cl, Ar, K, and Ca nuclei are all isotones of 20 because they all contain 20 neutrons. Despite its similarity to the Greek for "same stretching", the term was formed by the German physicist K. Guggenheimer by replacing the "p" in " isotope" with "n" for "neutron".

The largest numbers of observationally stable nuclides exist for isotones 50 (five; Kr, Sr, Y, Zr, Mo) and 82 (six; Ba, La, Ce, Pr, Nd, Sm). Neutron numbers for which there are no stable isotones are 19, 21, 35, 39, 45, 61, 89, 115, 123, and 127 or more. In contrast, the proton numbers for which there are no stable isotopes are 43, 61, and 83 or more. 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 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). Odd neutron numbers for which there is a stable nuclide and a primordial radionuclide are 27 (V), 65 (Cd), 81 (La), 85 (Sm), and 105 (Lu). Neutron numbers for which there are two primordial nuclides are 88 (Eu and Gd) and 112 (Re and Pt).