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Which elements can be diatomic and why?

Motivation

Hydrogen, Nitrogen, Oxygen and the Halogens tend to be thermodynamically stable as a diatomic molecule at room temperature, and are usually labelled as diatomic nonmetals. (Although I am unsure about Astatine). These are the ones you learn in school and I am fine with their reasoning. I want to know about other elements in particular.

Looking at the rest of the non-metals: Carbon, Phosphorus and Sulfur have diatomic molecules in gaseous states. All with a common theme of being heated at very high temperatures.

This leaves us with diatomic Selenium which I could not find anything on.

Further notes

Diatomicity doesn't seem to be just a non-metal thing:

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    $\begingroup$ I have a feeling almost every element can be found as a diatomic molecule in the right conditions (or at least present as a measurable mole fraction), though high temperatures and low pressures may be needed for many of them. In the majority of cases, any chemical bond will be more stable than no chemical bond. Plus there are some stranger cases, such as bound diatomic noble gas molecules, stable when encapsulated by a fullerene molecule. $\endgroup$
    – Nicolau Saker Neto
    Commented Jun 10, 2015 at 15:46
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    $\begingroup$ Alkali-earth metals and noble gases definitely can't form stable diatomic molecules, Zn/Cd/Hg likely can't too. Homoatomic bonds for most heavy non-transition metals are very weak so detecting diatomic molecules is unlikely. $\endgroup$
    – permeakra
    Commented Jun 10, 2015 at 15:56
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    $\begingroup$ @permeakra Diberyllium exists: 'diberyllium molecule exists (and has been observed in the gas phase). It nevertheless still has a low dissociation energy of only $59~\mathrm{kJ~mol^{-1}}$.' Also see here. $\endgroup$
    – bon
    Commented Jun 10, 2015 at 16:03
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    $\begingroup$ @bon Interesting, it shouldn't. Well, Palladium clasters shouldn't exist to, but they do. $\endgroup$
    – permeakra
    Commented Jun 10, 2015 at 16:28
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    $\begingroup$ Depends what you call diatomic - if you count helium vdV dimer than two atoms of any element which will have occasion and time will get bound. $\endgroup$
    – Mithoron
    Commented Jun 10, 2015 at 17:55

2 Answers 2

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This leaves us with diatomic Selenium which I could not find anything on.

Selenium forms a dimer with bond energy of 79 kcal/mol, according to table 8.6 (page 451) of Comprehensive Handbook of Chemical Bond Energies, which provides 5 references.

Tellenium dimer is 61 kcal/mol

Polonium dimer 45 kcal/mol

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In experiments where polyselenide and elemental selenium were heated with dienes to trap diatomic selenium into a diene complex, this complex was obtained with 5,6-dimethylene-cyclohexa-1,3-diene. See Ref. [1] for details.

Reference

  1. APA Chicago Harvard Rys, A. Z., Schultz, E. K. V., & Harpp, D. N. (2010). "Quest for diatomic selenium". Journal of Sulfur Chemistry, 31(5), 351–371. https://doi.org/10.1080/17415993.2010.513440
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