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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?

andselisk
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carbenoid
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    You know that Fr is itself unstable? – Mithoron Apr 13 '16 at 23:28
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    @Mithoron Yes, I'm aware that Fr decays quickly, but I suppose this is somewhat of a hypothetical question. – carbenoid Apr 13 '16 at 23:30
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    https://en.wikipedia.org/wiki/Francium says this superoxide would have more covalent character, the cited source doesn't say much more. – Mithoron Apr 13 '16 at 23:40
  • 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?

     – gannex Apr 14 '16 at 02:57
  • Was that the correct answer to the local exam? I also chose the superoxide. – Yunfei Ma Apr 16 '16 at 19:13
  • @YunfeiMa The correct answer is just the oxide $\ce{Fr2O}$ – carbenoid Apr 16 '16 at 19:24
  • Perhaps the question made a distinction between Oxides and Superoxides (considering oxide to be the formal name of the $O^{2-}$ ion) – Joseph Hirsch Dec 07 '16 at 23:35
  • Francium Oxide(Fr2O) is an ionic compound composed of Fr+ and O^2- ions. To balance the negative charge in the compound, 2 molecules of francium must be present for every 1 oxygen, meaning the compound must be Fr2O – Walker Schmidt Dec 07 '16 at 22:52
  • The claim is that heavier metals will combine with the superoxide anion $O^-_2$ – Joseph Hirsch Dec 07 '16 at 23:31
  • Related: http://chemistry.stackexchange.com/questions/32062/isolating-radium-oxide – Nilay Ghosh Dec 08 '16 at 04:42

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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 $\ce {Fr_2O}$.

Proscionexium
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