[This might belong in the physics forum.] I have the impression that when discussing stability of nuclides that there are two different usages of the term. 1. Atomic weight divided by nucleon count: The lower the number, the more stable is the nuclide. Iron ending up being the most stable. 2. Radioactivity: All nuclides that do not decay are called stable. Radioactive nuclide stability is measured by half life. Longer half life means more stable. Any way to resolve this?
I think for 1 you mean: ( isotope mass - (mass of 1 electron + mass of one proton) * Z - (mass of 1 neutron) * (A-Z) ) / A Which is the negative of the mass defect per nucleon. http://en.wikipedia.org/wiki/Binding_energy#Mass_defect
In any case, the point of my question is which description describes stability - the mass defect or radioactive half-life?
Depends what you mean by stability. A meter stick placed lengthwise on the ground is stable -- if you push it a little bit, it doesn't move much. A meter stick balanced on its end is unstable, meaning a small push will result in a drastic and significant change of state. No radioactive isotope is "stable" -- they fall apart all by themselves. A non-radioactive isotope with a very high mass defect per nucleon is more analogous to a meter stick on the ground lengthwise -- it's unlikely to fall far since there are relatively few places for it to fall to, even if pushed.
The mass defect indicates the binding energy between the neutrons and protons. The more bound they are, the more likely you've got a stable configuration. But not necessarily - you might have two sub-configurations that are each very tightly bound, but not to each other. Check out asymmetric fission. Since the half-life is an empirical measurement, IMHO it's a better description of how stable something is.
It doesn't work out that way. As atoms get larger, they need more neutrons than protons to remain stable. Elements with either too many or too few neutrons compared to protons are unstable, but the stability ratio changes with increasing atomic number.
That is the point of my question. You are mixing definition 2 (radioactivity) with definition 1 (binding energy).
