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there is a sentence "This isomer is unconfirmed" as a footnote explaining two isotopes. What do they mean by 'isomer' in this context?

Is it possible that an element may have different spatial distribution of neutrons and protons that could have an impact on its stability?

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    $\begingroup$ Follow the hyperlinked word of 'isomer' on the Wiki. Clearly in a chemical context this makes no sense, but science isn't always clearly defined between disciplines. In this instance there's something called a nuclear isomer. The 'm' more than likely refers to the word 'metastable', meaning that something is locally stable, but not globally. I'd recommend moving this to the Physics.SE. $\endgroup$ – user7232 Oct 23 '14 at 12:10
  • $\begingroup$ Since Glenn Seaborg won his Nobel prize in chemistry, perhaps one shouldn't be so hasty. That being said this is more nuclear physics. The $m$ does indicate a metastable excited nuclear state which will decay by $\gamma$ emission. This is analogous to an excited electronic state decaying by emitting a photon. $\endgroup$ – Jon Custer Oct 23 '14 at 13:01
  • $\begingroup$ Since I have a friend of the family who considers himself a nuclear chemist and worked on the table of isotopes, I think it's fine to keep it here in Chemistry.SE. $\endgroup$ – Geoff Hutchison Oct 23 '14 at 13:02
  • $\begingroup$ @TheJsyChemist I think that's a perfect answer if you can make into an answer (and dropping the migration suggestion). $\endgroup$ – Geoff Hutchison Oct 23 '14 at 13:03
  • $\begingroup$ I'll add my intuition where it's necessary, but, as a chemist, I believe it doesn't come under the remit of my expertise. I'm still not convinced that this is actually a chemistry question, in the politest way possible of course. $\endgroup$ – user7232 Oct 23 '14 at 13:32

The 'm' does indeed stand for metastable. The word 'isomer' in this sense refers to nuclear isomers, in which at least one of the nucleons is in an excited state. Think of it as somewhat analogous to an electron in an excited state, but occurring within the nucleus. Just like electrons release photons when an excited state decays to the ground state, when nuclear isomers decay to the ground state (which is different than 'normal' nuclear decay like $\alpha$ or $\beta$ decay), a gamma ray is release.

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