Can Nuclide Stability Be Determined by Atomic Weight or Radioactivity?

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    Definition Stability
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Discussion Overview

The discussion revolves around the concept of nuclide stability, specifically examining whether it can be determined by atomic weight or radioactivity. Participants explore different definitions and implications of stability, including theoretical and practical aspects related to nuclear decay and binding energy.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that stability can be defined in two ways: by the ratio of atomic weight to nucleon count, and by radioactivity measured through half-life.
  • One participant suggests that the lower the atomic weight divided by nucleon count, the more stable the nuclide, with iron being cited as the most stable example.
  • Another participant argues that stability primarily refers to how radioactive a nuclide is, or isn't.
  • It is noted that there is a concept of absolute stability, with helium-4 (4He) mentioned as an example of a nuclide that is considered absolutely stable due to its lack of half-life.
  • Some participants highlight that while certain nuclei may be theoretically unstable, their extremely long half-lives render them practically stable.
  • A distinction is made between the definitions of stability, particularly in the context of uranium-238 (U238) and its decay chain, where U238 is considered more stable than its decay products when viewed through the lens of radioactivity.
  • One participant suggests that the atomic weight ratio can be used to estimate the energy released in nuclear fission, linking it to binding energy.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of nuclide stability, with no consensus reached on whether atomic weight or radioactivity is the primary determinant of stability.

Contextual Notes

Participants acknowledge that the definitions of stability may depend on specific contexts and assumptions, and there are unresolved questions regarding the relationship between atomic weight, half-life, and practical stability.

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.
 

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