Avogadro constant

In chemistry, the Avogadro constant, commonly denoted NA, is a conversion constant or ratio between an amount of substance and the number of particles that it contains. The particles in question are any designated elementary entity, such as molecules, atoms, ions, or ion pairs. It is an SI defining constant with the exact value 6.02214076×1023 mol−1 (reciprocal mole). The numerical value of this constant when expressed in terms of the mole is known as the Avogadro number, commonly denoted N0. The Avogadro number is an exact number equal to the number of constituent particles in one mole of any substance (by definition of the mole), historically derived from the experimental determination of the number of atoms in 12 grams of carbon-12 (12C) before the 2019 revision of the SI, i.e. the gram-to-dalton ratio, g/Da. Both the constant and the number are named after the Italian physicist and chemist Amedeo Avogadro. The Avogadro constant is used as a proportionality factor to define the amount of substance n(X), in a sample of a substance X, in terms of the number of elementary entities N(X) in that sample: n ( X ) = N ( X ) N A {\displaystyle n(\mathrm {X} )={\frac {N(\mathrm {X} )}{N_{\mathrm {A} }}}} . The Avogadro constant NA is also the factor that converts the average mass m(X) of one particle of a substance to its molar mass M(X). That is, M(X) = m(X) ⋅ NA. Applying this equation to 12C with an atomic mass of exactly 12 Da and a molar mass of 12 g/mol yields (after rearrangement) the following relation for the Avogadro constant: NA = (g/Da) mol−1, making the Avogadro number N0 = g/Da. Historically, this was precisely true, but since the 2019 revision of the SI, the relation is now merely approximate, although equality...