Group 7: Halogens
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ASTATINE (or ‘What’s that element just under iodine?)
Many textbooks feature the predictions made by Dimitri Mendeleev about what he called ‘eka-silicon’, the element between silicon and tin in Group 4 of the Periodic Table. They also give a list of the properties of the element subsequently called germanium showing how well Mendeleev made his predictions.
Examination questions have, from time to time, invited candidates to ‘do a Mendeleev’ by predicting the properties of astatine, At, the element immediately below iodine in Group 7. It is assumed that astatine is entirely artificial and that nothing can be known about its properties. Neither of these assumptions is quite true although it is estimated that there is only about 50 mg of astatine in the whole of the Earth’s crust. Various isotopes of astatine have been produced by bombarding bismuth-209 with alpha particles.
A number of researchers have investigated the properties of astatine experimentally. A comparison of expectations and known properties is as follows.
1 Expected property: Like the rest of the halogens, astatine ought to be diatomic.
Actual property (Gmelin handbook 1985): Direct evidence for At2 is not strong but the ion At2+ has been identified.
2 Expected property: Chlorine, bromine and iodine react with alkalis:
Eg I2 + 2OH- <=> I- + IO- + H2O
Alkalis drive this reaction to the right, acids liberate the free halogen again. Astatine should behave in a similar manner.
Actual property: Astatine made by bismuth bombardment dissolves in alkalis but is re-precipitated on adding acid. This strongly suggests the reaction:
At2 + 2OH- <=> At- + AtO- + H2O
3 Expected property: Astatine should form ions At- in the same way as the other halogens but might be metallic enough to form a positive ion.
Actual property: Astatine can be reduced to At- by tin(II) ions or zinc. The ion At+ has been detected in mass spectrometry.
4 Expected property: The halogens form simple hydrides such as HI. HAt ought to exist.
Actual property: HAt has been detected by mass spectrometry.
5 Expected property: The halogens can be oxidised to halates(V) containing ions such as BrO3-. Astatine ought to do the same.
Actual property: Astatine has been oxidised by Ce(IV) in acid solution to the astatate(V) ion AtO3-.
6 Expected property: The oxidation state +7 has been made by oxidising lower oxidation states to eg IO4-. This ought to be possible with astatine also.
Actual property: XeF2 , a very powerful oxidising agent, has been reported to oxidise astatine to the ion AtO4-.
7 Expected property: Organic astatides ought to exist similar to the well known halogenoalkanes.
Actual property: Astatides of formulae CnH2n+1At have been prepared for n=2 to 6.
8 Expected property: Interhalogen compounds such as ICl can be prepared by direct combination of the elements. Astatine should form interhalogen compounds too.
Actual property: AtCl, AtBr and AtI have all been detected by mass spectrometry.
Note
I claim no originality for this article. It is based on a copy of one from several years ago, published in a journal aimed at Sixth Formers but I cannot remember which journal or the name(s) of the author(s).
If anyone recognises from this his or her own work I will gladly acknowledge and crave forgiveness.
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Chris Heron
Astatine oxidation states: in view of the increasing oxidising power of Tl(III), Pb(IV) and Bi(V) I suspect that Po(VI) and At(VII) may not exist at all [cf non-existence of FeO4 following increasingly oxidising V(V), Cr(VI) and Mn(VII)]. Any comments?
24 November 2004
updated: 27 August 2003
