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I've just begun reading about halonium ions and I noted that only the first four halogens are acknowledged to be able to form halonium ions, but hydrogen astatide can be formed.

I can't seem to find any answers on the internet, but knowing that astatine is super reactive and radioactive, I suppose it may be due to this that forming an "Astatonium" molecule would be nearly impossible?

On the Wikipedia entry of hydrogen astatide, it states that there is an Astatonium — listed as a conjugate acid. However, a quick Google search of the term Astatonium only reveals 14 results.

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    $\begingroup$ Maybe it could form such ions, but the fact that the half life of astatine is 8 hours makes it very hard to handle and to source, so it is not surprising that it's not a very studied halogen, from a chemist's point of view. Anyway, a computational chemist with some spare time could compare the energy involved in the formation of such ions to that of the other halogens, and predict if that would be thermodinamically possible. My guess is that it could do it: less polarizable atoms like F and Cl are the big deal, but Br and I (and At?) can acquire a positive charge quite easily $\endgroup$
    – The_Vinz
    Jan 3 '20 at 2:36
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    $\begingroup$ Related: chemistry.stackexchange.com/questions/75292/… $\endgroup$ Jan 3 '20 at 5:18
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    $\begingroup$ And chemistry.stackexchange.com/questions/48354/… $\endgroup$ Jan 3 '20 at 5:18
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No astatonium ion with hydrogen is known, but it appears that a cation $\ce{S7At^+}$ is formed by replacing a sulfur atom in $\ce{S8}$ with an isoelectronic $\ce{At^+}$ moiety (the actual oxidation state of astatine, with sulfur being more electronegative, is $+3$). The ion is proposed to explain the incorporation of astatine into the sulfur precipitate when a sodium thiosulfate solution is decomposed by acid in the presence of astatide ion.

Primary reference:

G. M. W. Visser, E. L. Diemer, Radiochimica Acta 33 145 (1983).

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