ACS: Avogadro’s out

Go to the profile of Katharine Sanderson
Mar 27, 2019
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Here’s a bit of gossip – Avogadro’s constant, the one that lets you work out how much is in a mole of something, is under threat from a bunch of physicists who want to see it declassified as an absolute number, and instead tied to Planck’s constant, which is altogether more complicated to explain but essentially is used in quantum mechanics to bunch things into packets, or quanta. Not very clearly explained, but I’m no physicist. Check out other definitions here and here.

The person who told me this shocking piece of news is a member of the ACS nomenclature committee. Before you all rush out and try to recalculate the number of moles in your morning coffee – don’t panic. My source tells me that on a practical day to day basis, there will be no change, although explaining moles to a tenth-grader will be more difficult if the change ever makes it through.

The paper that started it all was apparently published in the journal Metrologia, by Ian Mills, although I’m having trouble tracking down the paper.

From my brief conversation, it seems that the idea is to relate Avogadro’s number to Planck’s constant so that the number becomes a relationship between the two numbers rather than an exact number. The grandiose phrase I heard was that this would relate Avogadro’s number to the invariants of nature. What would happen in your world if suddenly you had to redefine Avogadro’s number? Anything? Nothing?


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