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?
i haven’t reader the paper yet. the constant is fundamental to statistical thermodynamics, which deals with a different type of problems from quantum mechanics does. how can they be related?
This paper?
https://www.iop.org/EJ/abstract/0026-1394/43/3/006
Full text: https://www.iop.org/EJ/article/0026-1394/43/3/006/met6_3_006.pdf
Relevant bits seem to be sections 2.2.4 and 4.1.4.
“The mole, unit of amount of substance of a specified elementary entity, which may be an atom, molecule, ion, electron, any other particle or a specified group of such particles, is such that the Avogadro constant is exactly 6.022 141 5 × 10^23 per mole.”
So as I read it, they are proposing changing from a relationship between two things (number of 12-C atoms in a kg of 12-C) to a fixed number. The implication is then that the (kilogram, ampere, kelvin and) mole are defined in terms of exactly-known physical constants (i.e. invariants). Currently the kilogramme is defined in terms of a certain lump of metal in Paris.
The mole will therefore change (in definition, but not in practice within margins of error) but Avogadro’s number remains.
There’s more on the redefining of Avogadro’s number – and the replacement of that lump of metal in Paris – here:
https://www.rsc.org/chemistryworld/Issues/2007/August/Redefiningthekilo.asp
Which constant would be more practical, Avogadro’s or Planck’s? Why?