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The mole is still defined as an SI unit representing the number of constituent particles (whether they be atoms, molecules etc.) in 12 grams of carbon isotope 12 (carbon-12).

Effective from May 20 this year the definition of the mole will change to a unit representing Avogadro constant number of particles.

Since the Avogadro constant was and will be still tied to the mole (number of particles in a mole), I think this also changed the definition of the Avogadro constant, though I'm not sure. Although the old and new definitions define the constant as the number of particles contained in the mole, it seems that the definition of the mole has changed from the earlier carbon-12 definition to simply the Avogadro constant.

First of all I would like to know if anyone knows why this redefinition happened now (it was decided at the end of the last year).

Also, please correct anything wrong I've said. This is rather confusing for me and the definitions seems rather circular. The new definitions are that Avogadro's constant is the amount of particles in a mole, and a mole is defined by the Avogadro constant.

Previous definition:
The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
2019 definition:
The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.02214076 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol−1 and is called the Avogadro number.
Avogadro constant - Wikipedia

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You have already mentioned the last definition of the mole before the proposed change:

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol”.

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According to this definition, the mole still depends on the definition of the kilogram. The main purpose of the proposed change is to find an independent definition. Another purpose is the possibility of linking the definition to an exact value (similar to the speed of light.)

The formal decision for the change was made during the 26th meeting (13–16 November 2018) of the General Conference on Weights and Measures (CGPM).

The motivation behind the change of the definition of the mole can, for example, be found in the resolutions of the 24th meeting (17–21 October 2011) of the General Conference on Weights and Measures (CGPM); however, the discussions and preparations started already much earlier.

On the possible future revision of the International System of Units, the SI

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considering

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that of the seven base units of the SI, only the kilogram is still defined in terms of a material artefact, namely, the international prototype of the kilogram (1st meeting of the CGPM, 1889, 3rd meeting of the CGPM, 1901), and that the definitions of the ampere, mole and candela depend on the kilogram,

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that it is also possible to redefine the mole so that it is linked to an exact numerical value of the Avogadro constant $N_\mathrm A$, and is thus no longer dependent on the definition of the kilogram even when the kilogram is defined so that it is linked to an exact numerical value of $h$, thereby emphasizing the distinction between amount of substance and mass,

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    $\begingroup$ Thank you. So mainly to decouple its definition from the kilogram. I personally find this a simpler definition. I also have many other questions such as why it wasn't just defined as Avogadro's constant in the first place, but I think there are already questions about that here. Actually I don't even quite get why this was a motivating force behind the decision exactly, the separation of the kilogram to the mole, but I'm sure I'll find out with some more reading. $\endgroup$ – Zebrafish Feb 28 at 16:56
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    $\begingroup$ In the old world, the Avogadro constant was the number of particles in a mole, which was determined by experiment to be approximately some numerical value. In the new world, the number of particles in a mole is the Avogadro constant, which is some numerical value by definition. If an experiment shows tomorrow that the number of atoms in 12 grams of carbon-12 is actually more or less than that number, then we will say that that sample of carbon is more or less than 1 mol, but not that the definition of the mole or the Avogadro number has changed. $\endgroup$ – hobbs Feb 28 at 20:39
  • $\begingroup$ Does anyone know why Carbon was chosen (as opposed to say the more 'obvious' choice of Hydrogen)? Perhaps gasses are too difficult to measure? $\endgroup$ – JosephDoggie Feb 28 at 21:45
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    $\begingroup$ @JosephDoggie, It is a very very long story as to why C-12 was chosen. Chemists and physcists used to have a different atomic mass references. Chemists fixed ordinary oxygen as 16 (exact), physicists chose oxygen -16 isotope. Note that ordinary O is a mixture of isotopes. All the time their reported values were off and every author had to tell whether they are using chemist's scale or a physical scale. Later two persons suggested why don't we all have a common choice i.e. C as 12 exact, this will cause minimum changes in atomic masses scale. Finally the feuding parties agreed to use C-12. $\endgroup$ – M. Farooq Feb 28 at 23:41
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    $\begingroup$ @M.Farooq wouldn't C also be easier to weigh and measure than O, with it being a solid? $\endgroup$ – JAD Mar 1 at 8:13

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