I read that the heavier alkali metals, like potassium, rubidium, and cesium all prefer to form superoxides. Since francium is the heaviest alkali metal so far, I assumed it would follow the same trend as the previous alkali metals, forming $\ce{FrO2}$. However, I was told that the most stable oxide of francium is $\ce{Fr2O}$. Why would this be the case?

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    $\begingroup$ You know that Fr is itself unstable? $\endgroup$
    – Mithoron
    Apr 13, 2016 at 23:28
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    $\begingroup$ @Mithoron Yes, I'm aware that Fr decays quickly, but I suppose this is somewhat of a hypothetical question. $\endgroup$
    – carbenoid
    Apr 13, 2016 at 23:30
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    $\begingroup$ en.wikipedia.org/wiki/Francium says this superoxide would have more covalent character, the cited source doesn't say much more. $\endgroup$
    – Mithoron
    Apr 13, 2016 at 23:40
  • $\begingroup$ All Alkali metal oxides below sodium are superoxides. This is because of the bigger alkali metal cations' extremely low charge density and the high polarizability of superoxide relative to peroxide and regular oxide. What this means in the real world, though, I do not know. I don't think Francium itself is stable, so what can we really say about the stability of its oxides? $\endgroup$
    – gannex
    Apr 14, 2016 at 2:57
  • $\begingroup$ Was that the correct answer to the local exam? I also chose the superoxide. $\endgroup$
    – Yunfei Ma
    Apr 16, 2016 at 19:13

1 Answer 1


With a half-life of just 22 minutes for the longest-lived isotope francium-223, Wikipedia says that due to intense heat of radioactivity it vaporizes itself. This is because as more and more atoms of francium clump together, the collective heat produced increases tremendously.

As the article states that $\ce{FrO2}$ is quite covalent, it couldn't withstand the intense heat that Fr atoms emit together from within $\ce{FrO2}$ and thus $\ce{Fr2O}$ which is more stable is favorable over $\ce{FrO2}$. This is the same reason we couldn't get an observable chunk of francium element in nature as of yet, either in free state or in combined state.

Look up this list of inorganic compounds that also mentions the existence of $\ce {Fr_2O}$.

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    $\begingroup$ Please don't post links to the mobile versions of the websites (unless there is a really good reason to do so). Note isotopes are usually denoted as "francium-223" or "$\ce{^{223}Fr}$", but not as "Fr-223" (this notation is often used for labeling and counting atoms). $\endgroup$
    – andselisk
    May 18, 2023 at 10:59
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    $\begingroup$ @andselisk The simple notation Fr-223 is widely used in radiochemistry and radiation protection (e.g. by ICRP); probably even more widely used than the somewhat unwieldy notation $\ce{^223Fr}$. $\endgroup$
    – Loong
    May 18, 2023 at 11:26
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    $\begingroup$ FrO2 woukd not convert to Fr2O. The radiant energy would blow the atoms completely apart. Francium samples need a big heat sink, which is usually supplied by making a highly dilute solid solution in the corresponding caesium compound. $\endgroup$ May 18, 2023 at 18:00
  • $\begingroup$ @OscarLanzi May I say that the environment in francium favors the formation of $\ce{Fr_2O}$ over $\ce{FrO_2}$ instead of saying that it converts? $\endgroup$ May 18, 2023 at 18:06
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    $\begingroup$ The environment in francium favors the sample being blown apart by radioactivity. Period. $\endgroup$ May 18, 2023 at 18:22

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