Classification Based on Nuclear Stability

Beyond classifying nuclides by how their proton/neutron counts relate to each other, they can also be split into two broad groups purely on the basis of stability.

A stable nuclide is one whose composition -- its numbers of protons and neutrons -- stays fixed indefinitely. The number and arrangement of the surrounding electrons can and does change (for instance during a chemical reaction, or on ionisation), but that has no effect on the nucleus itself. …