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What's the least common element in the universe? And what is its frequency measured in Hertz?

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    $\begingroup$ What do you mean by frequency? $\endgroup$ – Greg Nov 18 '20 at 1:21
  • $\begingroup$ Any element above 118, oganesson, in the periodic table. It's frequency is zero Hz, since it isn't here, yet. And there may be many others that haven' been discahvahd. (T, Lehrer) $\endgroup$ – DrMoishe Pippik Nov 18 '20 at 2:34
  • $\begingroup$ Probably astatine. $\endgroup$ – Nilay Ghosh Nov 18 '20 at 3:57
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    $\begingroup$ Questions without elaboration effort are not very welcome, and may be closed. Always think about and search for possible answers thoroughly before posting a question. Written effort attracts elaborated answers. Include in the question what you already know/thought about, what you found in textbook or online resources, what you have understood or not/tried/failed. Always add eventually relevant circumstances and background to prevent wrong assumptions and requests for clarifications. $\endgroup$ – Poutnik Nov 18 '20 at 6:31
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    $\begingroup$ There are 2 elements in universe that are so rare they have not been observed in the Universe yet. Which of them is less common ? $\endgroup$ – Poutnik Nov 18 '20 at 6:37
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(edited based on @JonCuster's comment) The rarest known elements are most probably the man-made heavy elements. These isotopes (elements) are very unstable. They most probably also formed in supernovas, but their natural formation is minimal, and due to their short lifetime, they do not accumulate either.

As Jon added, most probably heavier elements are also formed in supernovas, however, I am not aware of any of them observed. The reason for that is 1) again, short lifetime prevents any large accumulation, 2) under the formation conditions they are not atoms, but bare nuclei /plasma, so one cannot observe their atomic spectrum.

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    $\begingroup$ One might suspect there are a number we haven’t made, needing supernova conditions to appear even briefly... $\endgroup$ – Jon Custer Nov 18 '20 at 1:30
  • $\begingroup$ @JonCuster Yes, I guess. Why do not you make it an answer? $\endgroup$ – Greg Nov 18 '20 at 2:51
  • $\begingroup$ I'd just add it to your answer - as additional info around the 'formation in stars is minimal' and call it good. $\endgroup$ – Jon Custer Nov 18 '20 at 3:00

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