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B Does having more neutrons in an isotope make it more or les stable?

  1. May 20, 2016 #1
    Does more neutrons in an isotope make it more or less stable? I got two contradicting sources. I am also quite confused overall about the whole isotope business.

    The definition of an isotope is that it is an atom with the same number of protons but different number of neutrons.

    For example, carbon isotopes go from 8 to 22 as far as wiki says (https://en.wikipedia.org/wiki/Isotopes_of_carbon) and it also says 12C and 13C are both stable, and all others are unstable. But, first of all, isn't 12C not an isotope, since the number of protons is the same as number of neutrons?

    Secondly, it seems that there are unstable 8C, 9C and so forth and then 14C over to 22C must be unstable as well. So it seems to me that greater the number of neutrons doesn't exactly mean either more or less unstable, but rather that the closer the number of neutrons to protons the more stable the atom. Why would these books/internet/teachers try to say either of these incorrect statements?

    Thanks for help.
  2. jcsd
  3. May 20, 2016 #2


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    It is a carbon isotope. Isotope does not mean it cannot have the same number of protons and neutrons. Just that nuclei with a given number of protons are isotopes of the same element.

    What isotopes are stable varies with the number of protons. For relatively light elements, you need roughly the same number of protons and neutrons. For heavier elements there is usually more neutrons in the stable isotopes.

    Without you actually referencing a text, it is impossible to say whether it is the statement or your interpretation of it that id incorrect.
  4. May 20, 2016 #3


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    you can always call all the elements isotopes... the only difference between isotopes of an element is the number of neutrons... so 12C and 13C are carbon-isotopes; differing only to the number of neutrons.

    my above comment answers it.

    In general the number of neutrons act as a glue for heavier nuclei... That is the reason of the trend in the Nuclear Stability Curve (http://butane.chem.uiuc.edu/pshapley/GenChem1/L5/graph.gif) where you can see that the curve has a trend to move away from the Nprotons=Nneutrons.
    Adding neutrons makes the strong interaction stronger and the element more stable. The fewer neutron nuclei then are unstable because protons "don't really like" coexisting together alone. Adding blindly neutrons is not helping though, because the extra neutrons [being fermions] have to keep occupying higher energy levels of the nucleus [it's the reason of the 'asymmety' term in the Semi-empirical formula]... in other words you make your nucleus unstable with respect to beta- decay.
    For heavier nuclei the equality between Nprotons and Nneutrons is altered again for the same reason; although you have the asymmetry term you get larger coulomb interactions between the protons...
    So in general it's a competition between two extremes and the most stable one is the one that relatively stands in the middle.
  5. May 20, 2016 #4
    It's from a CGP book, quote ''In general, the greater the number of neutrons compared with the number of protons, the more unstable the nucleus.''
  6. May 20, 2016 #5


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    In general there are more isotopes with extra neutrons than isotopes with too few neutrons. The above statement is too vague to be useful.
  7. May 20, 2016 #6


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    That statement is wrong.
    It applies to nuclei with more neutrons than in the stable region, but limited like that it becomes trivial - the more you go away from the stable region, the shorter the lifetime gets. This is true for both directions.
  8. May 22, 2016 #7

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    If you are neutron-rich, adding more neutrons makes things worse. If you are neutron-poor, adding more neutrons makes things better.
  9. May 22, 2016 #8
    I guess there would be some elements for which there is no stable isotope, just some isotopes with a longer half life than most.
  10. May 22, 2016 #9


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    Everything above lead (82 protons) does not have any stable isotope, and lead-208 is the heaviest isotope where no decay has been observed.
    Zirconium-92 with 40 protons and 52 neutrons is the heaviest nucleus where no decay is possible at all. Everthing above that should be instable, but sometimes with lifetimes so long that the decay has not been observed yet.
  11. May 22, 2016 #10
    Is it possible to accurately predict the half life of an isotope in theory?
    I know that there are good reasons to propose that Isotope X should be more stable than isotope Y.
    However, can we reasonably well make a prediction for an isotope's half life, or is that only able to be ascertained by experiment.
  12. May 22, 2016 #11


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    Depends on what you call "accurately". For alpha and beta decays, it is often not hard to get it right within an order of magnitude, and better calculations can be better than a factor of 2. Making more accurate predictions is really challenging - nuclei are very complex. As soon as you can observe the decay, experimental values are more precise than theoretical expectations.
  13. May 23, 2016 #12
    Perhaps it was referring to fission products? There it is generally true that the fission products have an overabundance of neutrons.
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