The Wikipedia entry for density states

Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser.

I have not been able to turn up what these chemical compounds are. I can imagine a salt or some molecule in which dense atoms (e.g. of mercury) are packed in a conformation such that these atoms are held closer together than in the pure metal, so giving the molecule a higher density than the pure metal or of higher molecular weight metals like osmium.

I know there are elements with higher molecular weight than osmium and I have read about superdense exotic states of matter. I am curious about this chemical (presumably comprised of atoms with lower MW that osmium) which is more dense than osmium - presumably at earthly temperatures and pressures. Or an osmium-containing molecule more dense than metallic osmium would qualify.

If someone can answer this question he or she might also consider editing the Wikipedia article to clarify.

  • 1
    $\begingroup$ I read once that some alloys of osmium and iridium may be denser than either pure element but when I tried to find it again later, I failed. At least, there are non-exotic possibilities other than compounds. $\endgroup$ – badjohn Oct 31 '17 at 13:37
  • $\begingroup$ The core of the Sun is denser than osmium, but that is "cheating" because we use high pressure. $\endgroup$ – Oscar Lanzi Oct 31 '17 at 19:46

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