I've been observing the two-step decay of iodine-131 and can't quite determine what the asterisk above Xenon-131 indicates, for instance in the decay process according to Wikipedia:

decay process of Iodine-131 into Xenon-131, with an asterisk on the Xenon

I'm currently unsure as to what precisely the superscript asterisk on Xenon is. Apologies for the meekly idiotic inquiry, I'm not all too educated on this matter.


1 Answer 1


It means that the nucleus is not in its ground state but in an excited one. As such, Xe* can and does further decay into the ground state of Xe.

  • 2
    $\begingroup$ More precisely, since it is a nuclear reaction, the nucleus is not in the ground state. $\endgroup$
    – Jon Custer
    Commented Nov 25, 2018 at 18:22
  • $\begingroup$ You're much right, I'll correct my answer $\endgroup$
    – pp.ch.te
    Commented Nov 25, 2018 at 21:42
  • 2
    $\begingroup$ It may be worth noting that occasionally this means the first excited state in particular (with $\mathrm{Xe}^{**}$ meaning the second state and $\mathrm{Xe}^{***}$ the third, etc) but that that the same notation is often used in an inclusive manner to mean any excited state. Which meaning is intended must generally be extracted from the context. $\endgroup$ Commented Nov 25, 2018 at 22:48

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