PHYSICS AND CHEMISTRY: Avogadro constant

In chemistry and physics, the Avogadro constant (symbols: L, N_A) is defined as the number of constituent particles (usually atoms ormolecules) per mole of a given substance, where the mole (abbreviation: mol) is one of the seven base units in the International System of Units(SI). The Avogadro constant has dimensions of reciprocal mol and its value is equal to 6.02214129(27)×10²³ mol⁻¹. The constant happens to be quite close to an integer power of two, specifically only about 0.37% less than 2^79 mol⁻¹, making the latter a useful approximation in nuclear physics when considering chain reaction growth rates.

Previous definitions of chemical quantity involved Avogadro's number, a historical term closely related to the Avogadro constant but defined differently: Avogadro's number was initially defined by Jean Baptiste Perrin as the number of atoms in one gram-molecule of hydrogen. It was later redefined as the number of atoms in 12 grams of the isotope carbon-12 and still later generalized to relate amounts of a substance to their molecular weight. For instance, to a first approximation, 1 gram of hydrogen, which has a mass number of 1 (atomic number 1), has6.022×10²³ hydrogen atoms. Similarly, 12 grams of carbon 12, with the mass number of 12 (atomic number 6), has the same number of carbon atoms, 6.022×10²³. Avogadro's number is a dimensionless quantity and has the numerical value of the Avogadro constant given in base units.

The Avogadro constant is fundamental to understanding both the makeup of molecules and their interactions and combinations. For instance, since one atom of oxygen will combine with two atoms of hydrogen to create one molecule of water (H₂O), one can similarly see that one mol of oxygen (6.022×10²³ of O atoms) will combine with two mol of hydrogen (2 × 6.022×10²³ of H atoms) to make one mol of H₂O.

Revisions in the base set of SI units necessitated redefinitions of the concepts of chemical quantity and so Avogadro's number, and its definition, was deprecated in favor of the Avogadro constant and its definition. Changes in the SI units are proposed that will precisely fix the value of the constant to exactly 6.02214X×10²³ when it is expressed in the unit mol⁻¹ (see New SI definitions, in which an "X" at the end of a number means one or more final digits yet to be agreed upon).