Avogadro's number (N_A), approximately 6.022 × 10²³, is one of the most fundamental constants in chemistry and physics. It represents the number of constituent particles (atoms, molecules, ions, or other entities) contained in one mole of a substance. This constant is named after the Italian scientist Amedeo Avogadro, who in 1811 first proposed that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

The exact value of Avogadro's constant, as defined by the International Bureau of Weights and Measures since 2019, is exactly 6.02214076 × 10²³ mol⁻¹. This redefinition was part of the 2019 revision of the SI units, which tied fundamental constants to exact numerical values rather than physical artifacts. Understanding Avogadro's number is essential for converting between macroscopic measurements and the microscopic world of atoms and molecules.

Avogadro's number serves as a bridge between the atomic scale and the macroscopic world we can observe and measure. In stoichiometry, it allows chemists to calculate the exact number of atoms or molecules involved in a chemical reaction from measurable quantities like mass and volume. This is crucial for understanding reaction ratios and predicting product yields.