Is the Avogadros number hypothetical

Avogadro's constant

The Avogadro's constantNA. is a physical constant named after Amedeo Avogadro, which is called the number of particlesN per amount of fabricn is defined. It indicates how many particles (e.g. atoms of an element or molecules of a chemical compound) are contained in a certain amount of substance (namely one mole) of the respective material. It is defined as:[1]

$ N_A = 6 {,} 022 \; 141 \; 29 \; (27) \ cdot 10 ^ {23} \ \ mathrm {mol} ^ {- 1} $

According to the definition of the atomic mass unitu is the mass m of 6.02214129 (27) 1023 Atoms 12C in the basic state exactly 12 g. The molar masses of all other elements are related to carbon-12.

Historical and designation of the constant

Avogadro's constant is of great historical importance for proving that matter is made up of atoms. At the beginning of the 19th century, many scientists viewed atoms as hypothetical particles whose existence was unproven.[2] The certainty of its existence also comes from determining the Avogadro number with the help of various methods, all of which have given a consistent value.

The Italian physicist Amedeo Avogadro recognized as early as 1811 that equal volumes of different ideal gases contain the same number of molecules (Avogadro's law). In 1865, the Austrian physicist and chemist Josef Loschmidt succeeded for the first time in determining the size of molecules by order of magnitude. Ludwig Boltzmann named the number of molecules in one cubic centimeter of air derived from Loschmidt's results Loschmidt number. The number of particles per unit volume under normal conditions becomes Loschmidt's constantNL. called. The term Loschmidt number however, it is mistakenly v. a. in older German-language literature also synonymous with Avogadro number used.

It was not until 1909, after the deaths of Loschmidt and Avogadro, that the French chemist Jean-Baptiste Perrin suggested the number of particles in a mole as Avogadro number to call. The relationship between the Avogadro number in the SI system $ {} _ {\ left \ {N_A \ right \} _ \ mathrm {SI}} $ and the Avogadro constant $ {} _ {N_A} $ applies:

$ N_A = \ left \ {N_A \ right \} _ \ mathrm {SI} \, \ frac {1} {\ mathrm {mol}} $


The Avogadro constant NA. is used to convert between sizes that relate to individual particles and those that relate to amounts of substance measured in moles.


A precision method for determining the Avogadro constant is the XRCD method (English X-Ray Crystal Density). She uses X-ray diffraction experiments on single crystals to determine the size of the unit cell and the number of atoms it contains directly. The Physikalisch-Technische Bundesanstalt considers the results of these tests to be very promising and expects the kilogram and mole to be redefined soon.[3]


  • Peter Becker: History and progress in the accurate determination of the Avogadro constant. Rep. Prog. Phys., Vol. 64, 2001, pp. 1945-2008, doi: 10.1088 / 0034-4885 / 64/12/206.

Individual evidence

  1. ↑CODATA Recommended Values. National Institute of Standards and Technology, accessed June 16, 2011. Value for Avogadro's constant
  2. ^ History of atomic physics World of physics
  3. ↑The β€œnew” kilogram is getting closer. PTBnews 3/2010, German edition, December 2010; Retrieved August 13, 2011.