Avogadro's number and the mole concept

Avogadro's number is one of the fundamental constants. It is used by chemists to link the microscopic world of molecules to the macroscopic world of experiments, and by physicists to define the mass of objects from their elementary constituents.

In chemistry, we sometimes speak of large quantities of matter, sometimes of small ones. In practice, a chemist has to express a "chemical equation" in terms of atoms or molecules and convert it into terms of mass. Obviously, it's impossible to count the number of atoms or molecules contained in a sample of matter. Avogadro's number is used to count particles by measuring their mass. Closely linked to Avogadro's number is the notion of the mole, which links the microscopic world of atoms and molecules to the macroscopic world of grams, kilograms, and so on. Knowledge of Avogadro's constant thus enables molar masses to be determined and quantities of chemical species to be sampled.

In atomic and molecular physics, we use the atomic mass constant (symbol m _u ) or unified atomic mass unit (symbol u) :

with ${\mathcal{N}}_{\text{A}}$ Avogadro number, giving 1 u = 1.66054 × 10 ^–27 kg.

Avogadro's number, or Avogadro's constant, ${\mathcal{N}}_{\text{A}}$ , is the number of atoms or molecules (without interaction, at rest and in their fundamental state) per mole of pure substance, i.e. the number of ¹² C atoms contained in exactly 12 g of carbon 12. This number allows us to express the mass of a ¹² C atom, m( ¹² C), by the relation :