Why is it iron

Discussion in 'Chemistry' started by Beer w/Straw, Sep 9, 2020.

  1. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    Nuclear fusion occurs in a sun giving energy until iron.

    ? Yes I'm dumb.
     
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  3. exchemist Valued Senior Member

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    Fission too will result in iron or nickel, eventually. Fe and Ni happen to be the nuclei with the largest"binding energy", as shown by the well-known curve:-


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    (Potentially confusingly, the "binding energy" is the energy that has to be put in to break them apart. So it is the amount by which these nuclei have less energy than the corresponding separate nucleons.)

    As to WHY they are the most stable, I don't really know. Maybe someone with some nuclear physics can explain this properly.

    All I have is a hand-waving explanation that it is a balance between the attraction due to the strong force, which is apparently so short range that, for large nuclei, the outermost ones cease to be able to bind one another effectively right the way across the nucleus, versus the repulsion due to electrostatic charge, which obviously increases with atomic number. But it must be more complicated, I think, involving the shell model of the nucleus - which I know nothing about - in some way.
     
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  5. paddoboy Valued Senior Member

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    Pretty good explanation as is generally the case with Wiki....
    https://en.wikipedia.org/wiki/Nuclear_fusion
    subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifested as either the release or absorption of energy. This difference in mass arises due to the difference in atomic "binding energy" between the atomic nuclei before and after the reaction. Fusion is the process that powers active or "main sequence" stars, or other high magnitude stars.

    A fusion process that produces nuclei lighter than iron-56 or nickel-62 will generally release energy. These elements have relatively small mass per nucleon and large binding energy per nucleon. Fusion of nuclei lighter than these releases energy (an exothermic process), while fusion of heavier nuclei results in energy retained by the product nucleons, and the resulting reaction is endothermic. The opposite is true for the reverse process, nuclear fission. This means that the lighter elements, such as hydrogen and helium, are in general more fusible; while the heavier elements, such as uranium, thorium and plutonium, are more fissionable. The extreme astrophysical event of a supernova can produce enough energy to fuse nuclei into elements heavier than iron.

    In 1920, Arthur Eddington suggested hydrogen-helium fusion could be the primary source of stellar energy. Quantum tunneling was discovered by Friedrich Hund in 1929, and shortly afterwards Robert Atkinson and Fritz Houtermans used the measured masses of light elements to show that large amounts of energy could be released by fusing small nuclei. Building on the early experiments in nuclear transmutation by Ernest Rutherford, laboratory fusion of hydrogen isotopes was accomplished by Mark Oliphant in 1932. In the remainder of that decade, the theory of the main cycle of nuclear fusion in stars was worked out by Hans Bethe. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project. Fusion was accomplished in 1951 with the Greenhouse Item nuclear test.
     
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  7. paddoboy Valued Senior Member

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    The absorption of energy by Iron/Nickel is then the cause of eventual Supernova, and consequently even more hevier elements.
     
  8. paddoboy Valued Senior Member

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    Meaning that the real heaviest of elements are also forged in stellar remnant collisions of Neutron stars/Pulsars/White Dwarfs
     
  9. Seattle Valued Senior Member

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    Hydrogen fuses resulting in helium. That fuses to carbon. The chain ends with iron which is too stable to fuse further under normal circumstances.

    In stars larger than our Sun the during the sudden process of going Supernova the pressures are great enough to create all the other elements heavier than iron (gold, silver, etc).

    The lighter elements undergo fusion and the heavier elements undergo fission. It's easier to cause heavy elements to split and you can fuse the lighter elements.

    (This is just my layman's understanding and answer to your question..don't take it too literally).
     
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  10. exchemist Valued Senior Member

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    Good point about the elements heavier than Fe.

    According to this article: https://physicstoday.scitation.org/doi/full/10.1063/PT.3.3815

    there are actually two processes. Both involve capture of free neutrons, in an environment with a high neutron flux. One is a slow neutron capture process ("s-process"), possible in late stage evolution of stars with a mass 1-10 x that of the sun. The other is the one you refer to, involving a rapid capture process (the "r-process").

    It seems this can be achieved either in supernovae or in mergers between neutron stars. The article suggests the latter process is need to account for the full range of isotopes observed.
    There is a rather nice chart comparing the spectra of two late stage stars, one deficient in r-process (blue) and one with a marked r-process (red) : -

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  11. paddoboy Valued Senior Member

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    Not quite all.....as per post 5
    and.....

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    https://www.sciencealert.com/this-a...-shows-the-origins-of-every-atom-in-your-body
     
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