Avogadronummer

Avogadronummer, or Avogadro constant, is the number of elementary entities in one mole of a substance. Denoted N_A, its exact value is 6.02214076×10^23 mol^-1. The quantity serves as a bridge between the microscopic scale of atoms and molecules and the macroscopic scale used in chemistry and physics.

For any pure substance, one mole contains N_A particles. The mass of one mole, in grams, equals the substance's molar mass in grams per mole. For example, carbon-12 has a molar mass of 12 g per mole and contains N_A atoms. This relationship underpins stoichiometry and gas calculations.

Since 2019, N_A is defined as an exact constant by fixing its numerical value in the International System of Units (SI). The mole is defined as the amount of substance containing exactly N_A elementary entities; the kilogram is defined independently via the Planck constant, reinforcing a coherent set of units.

Historically, the constant is named after Amedeo Avogadro, who proposed in 1811 that equal volumes of gases at the same temperature and pressure contain the same number of particles. Measurements by Jean Perrin in the early 20th century established the reality of Avogadro's concept, leading to its adoption as a fundamental constant in the 20th century.

It remains central to calculations across chemistry, physics, materials science, and nanotechnology.