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According to the binding energy per nucleon vs mass number graph, it is observed that iron-56 has the maximum value of binding energy per nucleon ($\pu{8.75 MeV}$). It means that iron-56 is the most efficiently bound nucleus meaning that it has the least average mass per nucleon. This is the approximate basic reason why iron and nickel are very common metals in planetary cores since they are produced profusely as end products in supernovae and in the final stages of silicon burning in the stars. So, in one word, iron is quite stable.

Nucleon binding energy by number of nucleons in the nucleus of a given atom

But, what about helium and other noble gases? They are considered the most stable elements in the whole periodic table. But their binding energy per nucleon value is less than iron-56. So, they are not stable as iron-56. Is that true? Is iron the most stable element in the periodic table both structurally and chemically?

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    $\begingroup$ Chemical stability and the stability of the isotopes are two completely different topics. First is about the stability of the electronic shells and the second is about the stability of nucleus. $\endgroup$ – Greg Nov 9 '15 at 12:12
  • $\begingroup$ re: " This is the approximate basic reason why iron and nickel are very common metals in planetary cores, since they are produced profusely as end products in supernovae and in the final stages of silicon burning in the stars. " // That just isn't true. When the earth was very young it was molten. As it began to cool, then various phases started to separate. Fe/Ni formed a phase and since they are much denser that crustal compounds like SiO2 the Fe/Ni core "sank" to the center of the earth. $\endgroup$ – MaxW Nov 9 '15 at 22:45
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Yes, $^{56}\ce{Fe}$ has the most stable nucleus, and $\ce{He}$ is the most chemically inert element. These are different and unrelated qualities, pretty much like physical fitness and intelligence in a man. As for structural stability, there is no such thing in chemistry (there is one in architecture and another in mathematics, but those are out of scope of this question).

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  • $\begingroup$ What about the gold atom? I thought it's also very stable and hard to destroy. $\endgroup$ – Dr_Hope Feb 5 at 22:47
  • $\begingroup$ No more so than any other non-radioactive atom. $\endgroup$ – Ivan Neretin Feb 6 at 4:50
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No, nickel-62 is the most stable on a binding energy per nucleon basis. Fe-58 is second and Fe-56 is third.

See Fewell, M. P., "The Atomic Nuclide with the Highest Mean Binding Energy", Am. J. Phys., vol. 63, pages 653-658.

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  • $\begingroup$ The other half of this is a star burning the atoms via fusion. I'm too lazy to look up all the reactions, but the burning is exothermic to Fe and then becomes endothermic. It has to do with the mixture of isotopes created as the star goes from element to element. $\endgroup$ – MaxW Nov 9 '15 at 22:51
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    $\begingroup$ @MaxW, nickel-56 is the last energetically favorable fusion product in silicon burning (in a series of fusions with alpha particles). The nickel-56 then decays to iron-56. en.wikipedia.org/wiki/Silicon-burning_process $\endgroup$ – DavePhD Nov 10 '15 at 13:31
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Well, Iron has the most stable isotope(Iron-56), it has a large half-life which means it takes a fairly large amount of time to decay into the half of what was initially there. Noble gases have a low reactivity, which means a low electronegativity and a high ionization energy. Noble Gases should be identified as inert and not stable. They are not the same.

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    $\begingroup$ Large? As far as we know it has no half-life, unless proton itself decays. $\endgroup$ – Mithoron Oct 23 '17 at 17:39
  • $\begingroup$ Every element known to the humans have a half-life. Iron - 56 is stable but that's an iron isotope. Isotopes can be stable but there are many isotopes of iron that are radioactive and decay. For example, Iron-72 has a half-life of around 150 nanoseconds, while Iron-60 has a half-life of 2.62 X 10^6 which is a fairly LARGE amount of time. $\endgroup$ – Kirat Vyas Oct 25 '17 at 13:25

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