Is atomic weight universal?

Are we advanced enough to calculate the abundance of an element throughout the universe or is our atomic weight only based on the occurrence of an element in our planet alone?

• Our planet and even on our planet isotopic variations are observed. – MaxW Dec 27 '17 at 15:16
• @MaxW I suppose you meant "even on *other planets", thank you, but in outer space and throughout the other galaxies in our known universe? Do we consider isotope occurrences throughout there? Thanks for the comment! – Pre-alpha Dec 27 '17 at 15:27
• Atomic weights of the elements 2013 (IUPAC Technical Report) – Loong Dec 28 '17 at 11:59

2 Answers

There is no "or".

1. Yes, we are advanced enough to calculate the abundance of elements throughout the universe, albeit somewhat hypothetically. (But then again, the estimates here on Earth are also largely indirect. The radius of Earth is 6400 km, and the deepest borehole pierced only 12 km.)
2. Yes, if you look at the Periodic table on the wall of your chemistry class, the atomic weights listed there are only based on the isotope abundances in our planet alone.

True, the "earthly" and "Universe-based" atomic weights rarely differ by much. One notable exception is argon (earthly product is mostly $\rm^{40}Ar$, cosmic version is mostly $\rm^{36}Ar$). More subtle discrepancies are numerous. They are well-known, documented, and routinely used to prove that a particular rock is a visitor from space (even if nobody saw it fall).

So it goes.

• Thank you for the quick response, the conclusion that I'm arriving is: "Since we classify space rocks based on isotope occurrence. Does that mean for an universal atomic weight, we have to recalculate all the atomic weights separately as Universal Atomic Weight and Earthly Atomic Weight – Pre-alpha Dec 27 '17 at 15:49
• Like I said, they rarely differ by much. But if you want high precision, and if you intend to leave Earth and practice chemistry elsewhere in deep space, then yes, you'll need a whole new set of atomic weights. – Ivan Neretin Dec 27 '17 at 15:53
• Awesome exactly what I was so curious about, thanks for clarifying. Is it possible to quantify the amount by which they differ? – Pre-alpha Dec 27 '17 at 15:57
• I believe my example with Ar is by far the most extreme in terms of weight. Then again, weight is not very telling; isotope abundance is. Say, the earthly and cosmic fractions of $\rm^3He$ differ by an order of magnitude, but that doesn't mean much for averaged atomic weight, because both fractions are very tiny. – Ivan Neretin Dec 27 '17 at 16:06
• Maybe, but that's highly unlikely. For all we know, dark matter is not made of atoms. – Ivan Neretin Dec 28 '17 at 7:52

One notable example not mentioned above involves carbon. Carbon-14 on Earth is made continuously by the action of cosmic rays on nitrogen in the atmosphere. We take it for granted, but the implication is that carbon-14, which per se decays with a half-life of only 5700 years, is unlikely to be seen on a body without an appreciable amount of atmospheric nitrogen.

The effect on atomic weight, however, is very small. Carbon-14 is only a trace of Earth's naturally occurring carbon, even with all that atmospheric nitrogen floating around.

• Indeed, the atomic weight of carbon in usual materials varies between 12.0096 and 12.0116. However, this is mainly caused by the variability of the isotopic abundances of C-12 and C-13. – Loong Dec 28 '17 at 12:10