Avogadro constant

Avogadro constant or Avogadro number indicates the number of particles corresponding to one mole. Before the redefinition of SI units in 2019, it was defined as the number of atoms in 12 g of the pure carbon isotope 12C.

Carbon 12C contains 6 protons and 6 neutrons in the nucleus. If we assume that the mass of the proton and neutron is practically the same, and the mass of the electron is almost negligible compared to the them, we can say that Avogadro constant (approximately) tells how many nucleons weigh one gram.

The magnitude of Avogadro constant is fixed: 6,022 140 76 · 1023 mol−1 exactly.