a set of nuclides with equal proton number (atomic number), i.e., of the same chemical element but different neutron numbers, are called isotopes of the element. particular nuclides are still often loosely called "isotopes", but the term "nuclide" is the correct one in general (i.e., when "z" is not fixed). in similar manner, a set of nuclides with equal mass number "a", but different atomic number, are called isobars (isobar = equal in weight), and isotones are nuclides of equal neutron number but different proton numbers. likewise, nuclides with the same neutron excess ("n" - "z") are called isodiaphers. the name isotone has been derived from the name isotope to emphasize that in the first group of nuclides it is the number of neutrons (n) that is constant, whereas in the second the number of protons (p).