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I learnt that halogens always form covalent bonds to becoms diatomic molecules. So why is astatine monoatomic?

I mean they have the same properties, why shouldn't they all be diatomic?

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    $\begingroup$ Do you have a source claiming astatine is a monoatomic substance? $\endgroup$ – Nicolau Saker Neto May 15 '15 at 12:07
  • $\begingroup$ When I learn about properries of halogen, it said $\ce{At2}$. After that, my textbook say $\ce{At}$. $\endgroup$ – Simon-Nail-It May 15 '15 at 12:23
  • $\begingroup$ Is Fe, Al, B, or P monoatomic? $\endgroup$ – Georg May 15 '15 at 14:40
  • $\begingroup$ I think so. Do they have simlarity? $\endgroup$ – Simon-Nail-It May 15 '15 at 14:43
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    $\begingroup$ Phosphorus isn't monoatomic. It exists as either a polymeric substance, or as tetrahedral $\ce{P4}$ units, @Simon-Nail-It $\endgroup$ – Pritt says Reinstate Monica Jun 1 '17 at 7:02
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One key problem with astatine is that it's incredibly unstable. There are no known stable isotopes, and the longest-lived has a half-life of ~8 hours. So no one has been able to (yet) prepare enough to make real measurements. Thus, we don't know for sure whether solid At is diatomic (like the other halogens) or monatomic.

On the other hand, high-level first-principals (quantum) calculations have been performed that suggest astatine has some unusual properties. The full paper by Roald Hoffmann and Neil Ashcroft at Cornell was published in 2013 in Phys. Rev. Lett.: "Condensed Astatine: Monatomic and Metallic" and also covered in multiple science news sites, including Chemistry World.

The calculations on At:

predict that it will be a metallic solid. That’s perhaps not so surprising given that iodine, the halogen above it in the periodic table, is itself a dark silvery solid (although not truly metallic itself) and that astatine’s melting point is 302°C. But the real surprise of the new results is that the solid wouldn’t be composed of diatomic molecules, like all the other halogens, but would be monatomic.

Why would At be different than solid $\ce{I2}$?

... The monatomic form appeared only with spin-orbit coupling included. "These effects influence the atoms’ propensity to form certain bonds and crystal structures, usually by reducing molecular binding energies," says Hermann. Earlier calculations have indicated that in the gas phase astatine would still form a weakly bound diatomic molecule.

In other words, it's the spin orbit coupling contribution of relativistic effects that would make solid At monatomic.

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    $\begingroup$ I should point out that while I find the paper by Hoffmann and Ashcroft compelling, there's no experimental evidence for monatomic At yet. $\endgroup$ – Geoff Hutchison May 15 '15 at 15:40

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