I was taught that Avogadro's number was $6.023 \cdot 10^{23}$. Now, the accepted value is allegedly $6.022 \cdot 10^{23}$. Has there been a change? If so, when and why? There are still some sites that use $6.023 \cdot 10^{23}$, so if this is wrong, why have those sites not been corrected?


Currently, the definition of Avogadro constant depends on the definition of the kilogram, and thus has an uncertainty. The recommended value[1][2] is

$$N_\pu{A} = \pu{6.022140857(74) \cdot 10^{23} mol-1}.$$

It will probably be fixed, however, coming May 20th, 2019 on World Metrology Day to exactly[3][4]

$$N_\pu{A}^{2019} = \pu{6.02214076 \cdot 10^{23} mol-1}.$$

See the references for a thorough review of the history, reasons for redefinition, how some relations become exact and others inexact, and to which degree, etc.


[1] Marquardt, R.; Meija, J.; Mester, Z.; Towns, M.; Weir, R.; Davis, R.; Stohner, J. 'A Critical Review of the Proposed Definitions of Fundamental Chemical Quantities and Their Impact on Chemical Communities (IUPAC Technical Report)'. Pure and Applied Chemistry, 2017, 89 (7), 951–981. DOI: 10.1515/pac-2016-0808.

[2] Mohr, P. J.; Newell, D. B.; Taylor, B. N. 'CODATA Recommended Values of the Fundamental Physical Constants: 2014'. Rev. Mod. Phys., 2016, 88 (3), 035009. DOI: 10.1103/RevModPhys.88.035009.

[3] Marquardt, R.; Meija, J.; Mester, Z.; Towns, M.; Weir, R.; Davis, R.; Stohner, J. 'Definišion of the Mole (IUPAC Recommendation 2017)'. Pure and Applied Chemistry, 2018, 90 (1), 175–180. DOI: 10.1515/pac-2017-0106.

[4] 'A new definition of the mole has arrived'. IUPAC press release. archived link: goo.gl/5haPwJ


In fact it will get updated in the end of 2018. The new SI value will be updated to:$6.02214076×10^{23} \text{mol}^{-1}$


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