In my previous post I defined Isobars as "a line on a weather map or chart that connects areas of equal barometric pressure". This definition was from Meteorology. The examiner however, would be expecting the difference on the basis of Nuclear Physics given as below
Isotopes and Isobars
The various nuclides, or species, of a particular chemical element with equal proton number (atomic number), but different neutron numbers were called isotopes of the element, before the more inclusive term "nuclide" was internationally accepted (ca. 1950. Such particular nuclides may still be called "isotopes." However, nuclides with equal mass number but different atomic number are called isobars (isobar = equal in weight), whereas Isotones are nuclides of equal neutron number but different proton numbers.
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