Avogadro constant
The Avogadro constant ("N" or "L") is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample. Its SI unit is the reciprocal mole, and it is defined exactly as It is named after the Italian scientist Amedeo Avogadro. Although this is called Avogadro's constant (or number), he is not the chemist who determined its value. Stanislao Cannizzaro explained this number four years after Avogadro's death while at the Karlsruhe Congress in 1860. The numeric value of the Avogadro constant  expressed in reciprocal mole, a dimensionless number, is called the Avogadro number, sometimes denoted "N" or "N", which is thus the number of particles that are contained in one mole, exactly . The value of the Avogadro constant was chosen so that the mass of one mole of a chemical compound, in grams, is numerically equal (for all practical purposes) to the average mass of one molecule of the compound in daltons (universal atomic mass units); one dalton being of the mass of one carbon-12 atom, which is approximately the mass of one nucleon (proton or neutron). For example, the average  mass of one molecule of water is about 18.0153 daltons, and one mole of water ("N" molecules) is about 18.0153 grams. Thus, the Avogadro constant "N" is the proportionality factor that relates the molar mass of a substance to the average mass of one molecule, and the Avogadro number is also the approximate number of nucleons in one gram of ordinary matter. The Avogadro constant also relates the molar volume of a substance to the average volume nominally occupied by one of its particles, when both are expressed in the same units of volume. For example, since the molar volume  of water in ordinary conditions is about 18 mL/mol, the volume occupied by one molecule of water is about  mL, or about 30 Å (cubic angstroms). For a crystalline substance, it similarly relates its molar volume (in mol/mL), the volume of the repeating unit cell of the crystals (in mL), and the number of molecules in that cell. The Avogadro number (or constant) has been defined in many different ways through its long history. Its approximate value was first determined, indirectly, by Josef Loschmidt in 1865. (Avogadro's number is closely related to the Loschmidt constant, and the  two concepts are sometimes confused.) It was initially defined by Jean Perrin as the number of atoms in 16 grams of oxygen. It was later redefined in the 14th conference of the International Bureau of Weights and Measures (BIPM) as the number of atoms in 12 grams of the isotope carbon-12 (C). In each case, the mole was defined as the quantity of a substance that contained the same number of atoms as those reference samples. In particular, when carbon-12 was the reference, one mole of carbon-12 was exactly 12 grams of the element. These definitions meant that the value  of the Avogadro number depended on the experimentally determined value of the mass (in grams) of one atom of those elements, and therefore it was known only to a limited number of decimal digits. However, in its 26th Conference, the BIPM adopted a different approach: effective 20 May 2019, it defined the Avogadro number as the exact value , and redefined the mole as the amount of a substance under consideration that contains "N" constituent particles of the substance. Under the new definition, the mass of one mole of any substance (including hydrogen, carbon-12, and oxygen-16) is "N" times the average  mass of one of its constituent particles—a physical quantity whose precise value has to be determined experimentally for each substance. History. Origin of the concept. The Avogadro constant is named after the Italian scientist Amedeo Avogadro (1776–1856), who, in 1811, first proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of atoms or molecules regardless of the nature of the gas. The name "Avogadro's number" was coined in 1909 by the physicist Jean Perrin, who defined it as the number of molecules in exactly 32 grams of oxygen. The  goal of this definition was to make the mass of a mole of a substance, in grams, be numerically equal to the mass of one molecule relative to the mass of the hydrogen atom; which, because of the law of definite proportions, was the natural unit of atomic mass, and was assumed to be 1/16 of the atomic mass of oxygen. First measurements. The value of Avogadro's number (not yet known by that name) was first obtained indirectly by Josef Loschmidt in 1865, by estimating the number of particles in a given volume of gas. This value, the number  density "n" of particles in an ideal gas, is now called the Loschmidt constant in his honor, and is related to the Avogadro constant, "N", by where "p" is the pressure, "R" is the gas constant, and "T" is the absolute temperature. Because of this work, the symbol "L" is sometimes used for the Avogadro constant, and, in German literature, that name may be used for both constants, distinguished only by the units of measurement. (However, "N" should not be confused with the entirely different Loschmidt constant in English-language literature.) Perrin himself determined Avogadro's number by several different  experimental methods. He was awarded the 1926 Nobel Prize in Physics, largely for this work. The electric charge per mole of electrons is a constant called the Faraday constant and has been known since 1834, when Michael Faraday published his works on electrolysis. In 1910, Robert Millikan obtained the first measurement of the charge on an electron. Dividing the charge on a mole of electrons by the charge on a single electron provided a more accurate estimate of the Avogadro number. SI definition of 1971. In 1971, the International Bureau of Weights and Measures (BIPM) decided to regard the amount  of substance as an independent dimension of measurement, with the mole as its base unit in the International System of Units (SI). Specifically, the mole was defined as an amount of a substance that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12. By this definition, the common rule of thumb that "one gram of matter contains "N" nucleons" was exact for carbon-12, but slightly inexact for other elements and isotopes. On the other hand, one mole of any substance contained exactly as many molecules as one mole of any other substance. As a consequence  of this definition, in the SI system the Avogadro constant "N" had the dimensionality of reciprocal of amount of substance rather than of a pure number, and had the approximate value with units of mol. By this definition, the value of "N" inherently had to be determined experimentally. The BIPM also named "N" the "Avogadro "constant"", but the term "Avogadro number" continued to be used especially in introductory works. SI redefinition of 2019. In 2017, the BIPM decided to change the definitions of mole and amount of substance. The mole was redefined as being the amount of substance containing exactly  elementary entities. One consequence of this change is that the mass of a mole of C atoms is no longer exactly 0.012 kg. On the other hand, the dalton ( universal atomic mass unit) remains unchanged as 1/12 of the mass of C. Thus, the molar mass constant is no longer exactly 1 g/mol, although the difference ( in relative terms, as of March 2019) is insignificant for practical purposes. Connection to other constants. The Avogadro constant, "N" is related to other physical constants and properties.