2. Notion of mole

Long after Avogadro's death, the concept of the mole emerged from experimental observations that a quantity of molecules corresponding to its molecular mass contains the same number of molecules. This number corresponds to the eponymous Avogadro number (or Avogadro constant), noted ${\mathcal{N}}_{\text{A}}$ (or L, in German-speaking countries, in reference to Loschmidt).

The quantity used by chemists to specify the amount of chemical elements or compounds is now called "amount of matter". The quantity of matter is defined as being proportional to the number of elementary entities in a sample, the constant of proportionality being a universal constant identical for all samples, Avogadro's constant.

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