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?


2 Answers 2


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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