Reality → Matter → Molecules → Avogadro
The number of molecules in a mole is always 6 × 1023 [1] , regardless of the substance. This number is called Avogadro constant (or Avogadro number), in honor of Avogadro who was first to formulate the hypothesis that same volumes of all gases (at same pressure and temperature) contain the same amount of molecules [2] . The constant not only applies to all substances but also to all elementary entities, such as molecules, atoms, ions, electrons and other particles when the mole is used as a unit for the amount of substance. It is one of the most important constants to facilitate calculations and evaluations in molecular and physical chemistry. There are simple relations with the Gas, Boltzmann, and Faraday constants. The fact that they all fit nicely together and lead to several independent ways of calculating and verifying the Avogadro constant is one of the major proofs that molecules and atoms really do exist.
Loschmidt was first to estimate the number of molecules in a cubic centimeter of gas (Loschmidt constant), which allowed the determination of the Avogadro number.