Avogadro constant

The Avogadro constant NA is a named after Amedeo Avogadro physical constant, which is defined as the number N of particles per mole n:

It indicates the number of particles (such as atoms or molecules of a member of a chemical compound ) is contained in one mole of the substance. It is defined as:

Or about 602 billion billion particles. According to the definition of the atomic mass unit u is the mass m of 6.02214129 (27 ) · 1023 atoms 12C in the ground state exactly 12 g of carbon -12 is related to the molar masses of all other elements.

Historical and designation

The Avogadro constant has a great historical importance for the proof that matter is made of atoms. Many scientists looked at the beginning of the 19th century atoms as hypothetical particles whose existence is unproven. The certainty of their existence also comes from the determination of the Avogadro number using various methods, all of which have provided a matching value.

The Italian physicist Amedeo Avogadro recognized in 1811 that equal volumes of different ideal gases contain the same number of molecules ( Avogadrosches Act). With this law, he could explain measurements, which showed that in chemical reactions of gaseous substances, the volume ratio of the materials involved can be expressed by simple integers, formulated as Dalton's law of multiple proportions.

For the first time succeeded in 1865 the Austrian physicist and chemist Josef Loschmidt, to determine the size of molecules in order of magnitude. Ludwig Boltzmann called the derived results Loschmidt number of molecules in a cubic centimeter of air Avogadro's number. The number of particles per unit volume under normal conditions is called Avogadro 's constant NL. The term Loschmidt number but also mistakenly used esp. in older German literature as a synonym for Avogadro's number.

Only in 1909, after the death of Loschmidt and Avogadro suggested the French chemist Jean -Baptiste Perrin, to denote the number of particles in a mole as Avogadro's number. Between the Avogadro number in the SI system and the Avogadro constant, the relationship applies:


The Avogadro constant NA is used to convert between size specifications, which relate to individual particles, and those that relate to measured in mole quantities.

  • Faraday constant F and the elementary charge e:
  • Molar mass M and atomic mass ma:


For the determination of the Avogadro constant, there are about 60 independent methods. They may determine, inter alia, from the surface tension of dilute solutions, such as in the oil stain test, by radioactive decay, or also by the size of the elementary cubes of a crystal.

A precision method for the determination of the Avogadro constant is the XRCD method (English X - Ray Crystal Density). It uses X-ray diffraction experiments on single crystals in order to determine the size of the unit cell and the number of atoms contained in it directly. The Physikalisch- Technische Bundesanstalt rated the previous results of these experiments promise and anticipates an early definition of the kilogram and the mole is.