The question Difference between relative atomic mass, molar mass, and atomic mass unit? was just closed as duplicate. But for the change in SI definitions in May 2019, I would say of course.

However, the answers in the old question are pre-2019, so they are not up to date. Should we edit the top answer, saying that it was accurate between 1960 and 2019, and add a new answer? Or is there a different course of action someone would like to suggest?

  • 1
    $\begingroup$ In this particular case, the new question was closed as duplicate of Quick and simple explanation of molar mass, molecular mass and atomic mass, which is not affected by the 2019 redefinition and thus the upvoted answer (there's currently only one) does not need to be updated. Nevertheless, there are various other answers that really are affected by the change; therefore, this meta question is generally important. $\endgroup$
    – user7951
    Commented Sep 7, 2019 at 20:33
  • $\begingroup$ @Loong After I posed my question, I realized that your answer is not affected by the redefinition, as you say here. The new question, however, makes a statement that now is incorrect. Your excellent answer, of course, does not address this (you wrote it before the new question was asked...), so tagging the question as duplicate might have been unwise. $\endgroup$
    – Karsten Mod
    Commented Sep 8, 2019 at 16:41
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    $\begingroup$ Also see the Meta StackExchange tag for [obsolete-information]. $\endgroup$ Commented Sep 12, 2019 at 20:14

2 Answers 2


I personally prefer updating the existing answers to the best of our abilities. That was the course of action I chose when I realised there was a change in the definition of the oxidation state. I think this way we keep the information in the best place and avoid diluting it with links to other questions.
I think this is generally in the spirit of the site (and system).

I am positive that when Loong sees this posting he'll respond, or updates it.

If some things drastically change, we might consider a different approach though: We should mayhaps be asking the 'same' question again to allow for new(er) and updated answers. Then add a disclaimer on the outdated question and close it as a duplicate of the new target. Lastly put a historical lock on it to prevent change. That's as much archiving as we can do.

I firmly believe that textbooks will be updated eventually (except our notorious problem books maybe); but I guess that process is very slow. Keeping some version of the outdated information with the statement that it is now outdated is therefore something we should will benefit from. Otherwise we might run into the problem of new questions asking about old stuff (I'm looking at you d-orbital-hybridisation) again and again, because it will probably remain popular, and having to repeat the same stuff over and over and over again.


A slippery slope. If stack exchange was around earlier, would we also be changing all the definitions of the mole that were based on the relative atomic mass of oxygen to the definition based on carbon-12? See also some discussion at How to ask and answer nomenclature questions? about a similar issue.

Science, definitions, and understandings are changing all the time - bit by bit. If we knew all the answers already why would we study it or need new books etc?


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