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imyourzer0 t1_je3zxzb wrote

I recall reading elsewhere that the elements themselves (not compounds, per se) are distributed throughout the observable universe according to Zipf’s law (or something like it), so that they get less common as the atomic number increases. So, would it be a reasonable extrapolation to estimate compounds’ prevalence, then, by the chemical reactivities of their forming elements?


adamginsburg t1_je4qfp5 wrote

Zipf's law is a continuous power law distribution; while it's a good approximation when we don't know much, and therefore describes a ton of nature to the accuracy that we can measure it, it's not the best we can do with elemental abundances. Elements do something funnier; see the figures on


imyourzer0 t1_je4sktv wrote

Ah right! I think I mushed two youtube videos from one channel together in my memory. The second part of my question was really what I was interested in, though. Given some distribution of the elements across the universe, can we estimate the prevalence of the compounds they form, based on the elements' reactivities? For instance, this would predict that hydrocarbons should be common, since hydrogen is extremely prevalent and carbon is extremely chemically reactive?


adamginsburg t1_je53yx7 wrote

Yes, but. Reactivities depend greatly on physical conditions. In a cloud that's 100K, the molecular composition will be totally different than a cloud that's 1000K or one that's 10K. To predict which molecules form, you need a good census of how much gas is in each phase. We can actually tell that pretty well in galaxies by looking at various molecular and atomic emission lines.

The limit is actually in our knowledge of the chemistry, though. While we have good models to predict simple molecules, like CO, CO2, H2O, etc., we have a hard time with more complex molecules because the chemical reaction networks get very complicated, and in many many cases, the reaction coefficients are unknown. For example, our knowledge of Sulfur-containing molecular chemistry is very poor - there are too many reactions that haven't been measured in the lab, so we don't know what to predict. There is a lot of work left to be done in astrochemistry!


imyourzer0 t1_je61740 wrote

I certainly don't know this bit, but I would assume that more complex molecules (which from what you're saying we know less about) are exponentially less likely. I say this mostly because the probability of finding element 1 and element 2 at some point in the universe is certainly less than the probability of finding just 1 or just 2. So, once you've dealt with all the combinations of two or three, whatever's left is unlikely to severely tilt the scales, unless that numbers game really reverses under some conditions. But, I take your point that if we can't describe larger molecules well, it's hard to say whether something more has its finger on the scale. Thanks for the answers!


adamginsburg t1_je66y45 wrote

Less likely, yes. Exponentially less, no. You'd be roughly right if molecule formation was just a matter of random chance associations, which is true in the diffuse ISM, but it is not true in dense clouds where molecules form. A large fraction of all molecules form in clouds that get cold enough that the molecules stick to the surfaces of solid (dust) particles. Once they're there, they're in rich company: there's hydrogen, carbon, oxygen, etc. in abundance - and then more "normal" chemical processes (i.e., things you might find happening on Earth) start to take over. So yes, the numbers game starts to reverse pretty hard!

Purely from gas-phase processes, though, you're basically right; we expect that most molecules with >5 atoms rarely form in the gas phase. We usually draw the line at methanol, CH_3OH, which is a bottleneck in the formation of more complex molecules.


imyourzer0 t1_je68is5 wrote

That’s pretty wild! Like we’ve all heard from Sagan that we’re made of star dust, but I never really thought I ought to be emphasizing the dust part much!

I’ve got a whole bunch more questions, but I’ll spare you—you’ve been more than patient enough.