Isotone Isotone Two nuclides are isotones if they have the 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 changing the "p" in "isotope" from "p" for "proton" to "n" for "neutron". The largest numbers of observationally stable nuclides exist for isotones 50 (five: Kr, Sr, Y, Zr,
Valley of stability atomic and neutron numbers, forming what has been called an island of stability outside of the valley of stability. All atomic nuclei are composed of protons and neutrons bound together by the nuclear force. There are 287 different nuclei that occur naturally on earth, each corresponding to a unique number of protons, called the atomic number, Z, and a unique number of neutrons, called the neutron number, N. The mass number, A, of a nuclide is the sum of atomic and neutron numbers, A=Z+N. Not all nuclides are stable, however. According to Byrne, stable nuclides are defined as those having