facet isobar (nuclide) true true isobar 2019-05-09 Moved under classification datum. [Allotrope] isobar (nuclide) Isobar is a classification facet that classifies atoms (nuclides) of different chemical elements that have the same number of nucleons. Correspondingly, isobars differ in atomic number (or number of protons) but have the same mass number. [Wikipedia] classification datum