isotopic Definition

any of two or more species of atoms of a chemical element with the same atomic number and nearly identical chemical behavior but with differing atomic mass or mass number and different physical properties

nuclide

isospin

a quantum characteristic of a group of closely related subatomic particles (such as a proton and a neutron) handled mathematically like ordinary spin with the possible orientations in a hypothetical space specifying the number of particles of differing electric charge comprising the group —called also isotopic spin