Can Nuclide Stability Be Determined by Atomic Weight or Radioactivity?

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mathman
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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?
 

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As far as I've ever heard it discussed, stability refers to how radioactive something is. (Or isn't)
 
Your #1 is just an approximate way of estimating #2.

There is a notion of absolute stability. E.g., as far as we know, 4He is absolutely stable. It doesn't have a half-life at all.

There are also lots of nuclei that are theoretically unstable with respect to processes like proton emission, but the half-lives are so ridiculously long that they may as well be stable.
 
bcrowell said:
Your #1 is just an approximate way of estimating #2.

There is a notion of absolute stability. E.g., as far as we know, 4He is absolutely stable. It doesn't have a half-life at all.

There are also lots of nuclei that are theoretically unstable with respect to processes like proton emission, but the half-lives are so ridiculously long that they may as well be stable.

No. 1 and no. 2 are quite different. For example the U238 decay chain. From the point of view of the radioactive decay definition, U238 is far more stable than anything on the chain before the end (Pb206). However using the mass defect (No. 1) the species down the chain (after each alpha emission) are more stable than their predecessors.
 
It looks to me like #1 can be used to determine the energy released in a fission of the nucleus, aka the Binding Energy. The bigger the difference the more energy that will be released I would guess.