Avogadro constant

The number of entities in a mole of those entities. In effect this means, for example, the number of atoms in a quantity of atoms whose mass in grams is numerically equivalent to the atom’s atomic weight (or in the case of molecules, the number of molecules and the molecular weight). Symbol, N or N_A and sometimes L.

According to the 2002 CODATA recommended values, its value is 6.022 141 5 × 10²³ per mole with a standard uncertainty of 0.000 000 1 × 10²³.

It is named for Amadeo Avogadro (1776–1856). See mole.

At its 24ᵗʰ meeting (Paris, October 2011), the CGPM decided to declare in advance its intention to make the value of the Avogadro constant a matter of definition, rather than something to be determined experimentally. The new value will be exactly 6.022 14X × 10²³ entities per mole, where X stands for one or more yet to be determined digits. The new definition will not be adopted before 2014.

Peter Becker et al.
Determination of the Avogadro constant via the silicon route.
Metrologia, vol. 40, pages 271-287 (2003).