In nuclear physics and nuclear chemistry, the various species of atoms whose nuclei contain particular numbers of protons and neutrons are called nuclides. Nuclides are also characterized by their nuclear energy states (e.g., metastable nuclide 242mAm). Each nuclide is denoted by the chemical symbol of the element (this specifies Z) with the atomic mass number as a superscript. Hydrogen (H), for example, consists of one electron and one proton. The number of neutrons in a nucleus is known as the neutron number and is given the symbol N. The total number of nucleons, protons, and neutrons in a nucleus are equal to Z + N = A, where A is called the atomic mass number.
Isotones are nuclides with the same neutron number and are therefore different elements since they must differ in the number of protons. Therefore, isotones must be different chemical elements. For example, carbon – 13 and boron – 12 are isotones since both nuclei contain 7 neutrons. The name isotone has been derived from the name isotope by changing the “p” in “isotope” from “p” for “proton” to “n” for “neutron.”
The element’s nuclear properties (atomic mass, nuclear cross-sections) are determined by the number of protons and the number of neutrons (neutron number). For example, actinides with odd neutron numbers are usually fissile (fissionable with slow neutrons), while actinides with even neutron numbers are not fissile (but fissionable with fast neutrons). Due to the Pauli exclusion principle, heavy nuclei with an even number of protons and an even number of neutrons are very stable thanks to the occurrence of ‘paired spin.’ On the other hand, nuclei with an odd number of protons and neutrons are mostly unstable (i.e., radioactive). Neutron numbers with no stable isotones are 19, 21, 35, 39, 45, 61, 89, 115, 123, and 127 or more.
See also: Magic Numbers