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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.

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    $\begingroup$ 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
  • $\begingroup$ I edited the wikipedia entry. It now reads "Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds[citation needed] may be denser. " $\endgroup$ – Karsten Theis Sep 19 '19 at 13:27
  • $\begingroup$ As the set of chemical elements is just a subset of the set of all chemical substances, therefore maximal density within the elements is less or equal to maximal density within all substances. It was said "may", not "are". $\endgroup$ – Poutnik Jan 17 at 18:20
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This text was added as part of a refactoring/cleanup of the page in this edit in March 2013, where it replaced the text "Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but not the densest materials." This older text was tagged with the note "Which are denser, at standard conditions?" It in turn was added in this edit in May 2011 by an anonymous editor (the IP address used is apparently registered to the Wyoming Community College Commission). The original wording, "Osmium is [...] not the densest material", is ambiguous, but could be read as referring to exotic forms of matter such as neutronium, in spite of the earlier "standard temperature and pressure" caveat.

In short, this likely derives from a well-meaning but misguided editor attempting to note the existence of much higher-density, non-chemical substances in an inappropriate place.

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