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Water is a very common substance, not just on Earth, but also in space. Although water usually occurs in space as a solid and occasionally as a gas and relatively rarely as a liquid, the most common compound is H₂. I believe based on the abundance of elements in the universe (ignoring helium because it is a noble gas) that water is the most common multielemental (having more than one type of element in its structure) compound. Are there more common ones, or is water the most common one?

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    $\begingroup$ Because space is so empty, there's a fair chance the most common compound has only two atoms. Things like $\ce{HO}$ and $\ce{CH}$ can last for a very long time if there's nothing else to react with. $\endgroup$ – Nicolau Saker Neto Jun 6 '19 at 22:05
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    $\begingroup$ Ignoring He isn't good idea. HeH+ cation is one of most common in the Universe. $\endgroup$ – Mithoron Jun 6 '19 at 22:10
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    $\begingroup$ Helium is not only a noble gas, but also the least reactive one. Mithoron might be right, but I am doubtful though I think it is a possibility. $\endgroup$ – Number File Jun 6 '19 at 22:23
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    $\begingroup$ Well. if you ask only about neutral molecules then OK, but $\ce{HeH+}$ is, along with $\ce{H3^+}$, very common in space, so I think you should decide whether the question should be less or more broad. $\endgroup$ – Mithoron Jun 6 '19 at 22:37
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    $\begingroup$ Related, though I can't make any guarantees about the source: reddit.com/r/askscience/comments/3mr1i7/… $\endgroup$ – Tyberius Jun 6 '19 at 22:52
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You are indeed correct that the elemental molecule $\ce{H2}$ is the most abundant molecular species in the universe by far.[1]. In order to answer your question

Is water the most common multielemental compound?

I only focus on neutral species, of which around $50\%$ are in molecular, i.e. multiatomic form.[2]

In this category, second only in total abundance to molecular hydrogen $\ce{H2}$ is carbon monoxide $\ce{CO}$.

This has become evident during the extensive work that has been conducted in the last few decades as large-scale $\ce{CO}$ surveys of the Milky Way galaxy and other galaxies have been performed, further corroborating $\ce{CO}$ as the second most abundant neutral species and technically[3] as most abundant interstellar compound.

To conclude, it might be interesting to realize how substantial the difference in mass is still between the two most abundant molecular species in interstellar space:[2]

Estimates of the galactic mass of $\ce{H2}$ and $\ce{CO}$ can now be given with reasonable accuracy $m(\ce{H2}) = 2 \cdot 10^9 \; M_\odot$, and $m(\ce{CO}) = 2 \cdot 10^6 \; M_\odot$.

So in order to estimate $m(\ce{H2})$ which constitutes the bulk of the gas mass, carbon monoxide $\ce{CO}$ is typically used as tracer or proxy for $\ce{H2}$, as further reading I suggest a very informative post on the physics.SE: Why is $\ce{CO}$ a good tracer for $\ce{H2}$? How are those molecules correlated?

The mass unit $M_\odot$ is commonly used in astrophysics and represents the mass of the Sun, i.e. $M_\odot \cong \pu{1.98E30 kg}$.

The water molecule $\ce{H2O}$ is probably next, i.e. in third place, of the most abundant molecular (neutral) species in the universe which can be directly underpinned by the fact that it made up of the first and third most abundant elements in the universe, hydrogen $\ce{H}$ and oxygen $\ce{O}$*[4]. But I am not aware of any reputable source that explicitly states or confirms this assumption.

Footnotes & References

  1. vide, e.g. Why is $\ce{CO}$ a good tracer for $\ce{H2}$? How are those molecules correlated? or Wikipedia: List of interstellar and circumstellar molecules respectively

  2. Winnewisser, G. The chemistry of interstellar molecules. Topics in Current Chemistry,Cosmo- and Geochemistry 1981, 39–71. DOI: 10.1007/3-540-10920-X_14.

  3. vide ut supra, comment by Karsten Theis:

    $\ce{H2}$ is not a compound, it is an element. It is a molecule rather than an atom, though.

  4. van Dishoeck, E. F.; Herbst, E.; Neufeld, D. A. Interstellar Water Chemistry: From Laboratory to Observations. Chem. Rev. 2013, 113 (12), 9043–9085. DOI: 10.1021/cr4003177.

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    $\begingroup$ Hello! Chemical formulae should be typeset in upright, not italic. On Chemistry.SE this is most readily accomplished using the \ce{} macro provided by mhchem (see my edit for examples). $\endgroup$ – orthocresol Sep 10 at 22:11
  • $\begingroup$ @orthocresol I see, thank you very much for your edit and leaving a comment! I inherently thought about 'emphasising' the molecular formulae somehow, which I attempted ad hoc with the math environment \$...\$. The \ce{} approach looks better and it still does set off the chemical formulae from the text itself. I'll apply it in future posts. I hope the rest of the layout does conform with any Chemistry.SE standards (as I was playing around with the 'References & Footnotes' section at the end. $\endgroup$ – Diazenylium Sep 10 at 22:16
  • $\begingroup$ May I ask @orthocresol how one can set text in the grey background as you have done in the first comment? $\endgroup$ – Diazenylium Sep 10 at 22:31
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    $\begingroup$ The rest of your layout is more than fine. The grey-background text can be done by surrounding text with backticks `. $\endgroup$ – orthocresol Sep 10 at 23:22

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