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It's not a fundamental constant. > the Avogadro number was initially defined by Jean Baptiste Perrin as the number of atoms in one gram-molecule of atomic hydr

by Zarel 8y ago

It's not a fundamental constant. > the Avogadro number was initially defined by Jean Baptiste Perrin as the number of atoms in one gram-molecule of atomic hydrogen, meaning one gram of hydrogen. (from Wikipedia) (It's since been refined to be 12 grams of carbon-12.) So a mole is defined to be approximately one gram worth of protons and neutrons. We use it because grams are a significantly easier unit of mass for humans to work with, than like individual particles.

garmaine 8y ago
Read the OP. It is now a fundamental constant. (But you’re right if you’re referring to the status quo before).
- Zarel 8y ago
  No, it's not. Avogadro's number is a constant, but it's not fundamental - the only difference is that it's defined in terms of fundamental constants (namely, the Planck constant) now. It's still an arbitrary number defined in terms of the mass of a gram.
  - garmaine 8y ago
    Nope, it is fixed to be N_A = 602,214,076,000,000,000,000,000 exactly (6.02214076x10^23 to units precision). It used to be the case that the mole was an experimental value equal to the number of atoms in a certain mass of a certain something. That is no longer the case with this revision. It is a fixed, never changing integer constant. This does mean that 1 mole of carbon-12 is no longer exactly 12 grams. But it is approximately 12.0000000 grams, which is within the best we can experimentally measure today, so nothing changes in practice as a result of this update except first chapter of an introductory chemistry textbook (good excuse to push out a 9th edition for $250!). Therefore it is accurate to say now that whereas before the Avogadro's number was experimentally determined based the exact expressed mass of a carbon-12 atom relative to a platinum-iridium cylinder in Paris (the old kg), it is now the case that the expressed mass of the carbon-12 atom must be measured relative to the a kg definition based on Planck's constant. (I say "expressed mass" because this situation is a little confusing... I'm talking about the numbers we write down expressing the mass as a multiple of some standard kilogram. That reference mass changed, not the actual inertial mass.) EDIT: Or you can just read the draft of the agreement that was voted on. The definitions are on the first page: https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN...
    - Zarel 8y ago
      No, that's not the part we disagree about. Yes, I know, Avogadro's number is a fixed constant now. It's not a _fundamental constant_: https://en.wikipedia.org/wiki/Fundamental_constant https://en.wikipedia.org/wiki/Fundamental_constant A fundamental constant isn't just any fixed constant. It's specifically a constant that describes a fundamental property of the universe. For instance, c describes the speed of light in a vacuum, and is a fundamental physical property of the universe. Avogadro's constant isn't the same thing. It's just a number that humans decided would be useful. We could have fixed it to any other number; there's nothing fundamental about the number ~6.022e23.
      - garmaine 8y ago
        Fair enough. I was using a different definition, but arguing over definitions isn't a good use of time :)

by Zarel 8y ago

garmaine 8y ago

Read the OP. It is now a fundamental constant. (But you’re right if you’re referring to the status quo before).

Zarel 8y ago
No, it's not. Avogadro's number is a constant, but it's not fundamental - the only difference is that it's defined in terms of fundamental constants (namely, the Planck constant) now. It's still an arbitrary number defined in terms of the mass of a gram.
- garmaine 8y ago
  Nope, it is fixed to be N_A = 602,214,076,000,000,000,000,000 exactly (6.02214076x10^23 to units precision). It used to be the case that the mole was an experimental value equal to the number of atoms in a certain mass of a certain something. That is no longer the case with this revision. It is a fixed, never changing integer constant. This does mean that 1 mole of carbon-12 is no longer exactly 12 grams. But it is approximately 12.0000000 grams, which is within the best we can experimentally measure today, so nothing changes in practice as a result of this update except first chapter of an introductory chemistry textbook (good excuse to push out a 9th edition for $250!). Therefore it is accurate to say now that whereas before the Avogadro's number was experimentally determined based the exact expressed mass of a carbon-12 atom relative to a platinum-iridium cylinder in Paris (the old kg), it is now the case that the expressed mass of the carbon-12 atom must be measured relative to the a kg definition based on Planck's constant. (I say "expressed mass" because this situation is a little confusing... I'm talking about the numbers we write down expressing the mass as a multiple of some standard kilogram. That reference mass changed, not the actual inertial mass.) EDIT: Or you can just read the draft of the agreement that was voted on. The definitions are on the first page: https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN...
  - Zarel 8y ago
    No, that's not the part we disagree about. Yes, I know, Avogadro's number is a fixed constant now. It's not a _fundamental constant_: https://en.wikipedia.org/wiki/Fundamental_constant https://en.wikipedia.org/wiki/Fundamental_constant A fundamental constant isn't just any fixed constant. It's specifically a constant that describes a fundamental property of the universe. For instance, c describes the speed of light in a vacuum, and is a fundamental physical property of the universe. Avogadro's constant isn't the same thing. It's just a number that humans decided would be useful. We could have fixed it to any other number; there's nothing fundamental about the number ~6.022e23.
    - garmaine 8y ago
      Fair enough. I was using a different definition, but arguing over definitions isn't a good use of time :)