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In chemistry and physics, the Avogadro constant (symbols: , ) is the number of constituent particles, usually atoms or molecules, that are contained in the amount of substance given by one mole. Thus it is the proportionality factor that relates the molar mass of a compound to its mass. Avogadro's constant has the value in the International System of Units (SI). 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 atomic hydrogen, meaning one gram of hydrogen. This number is also known as Loschmidt constant in German texts. The constant was later redefined as the number of atoms in 12 grams of the isotope carbon-12 (12C) and still later generalized to relate amounts of a substance to their molecular weight. For instance, to a first approximation, 1 gram of hydrogen element (H), having the atomic (mass) number 1, has hydrogen atoms. Similarly, 12 grams of 12C, with the mass number of 12 (atomic number 6), has the same number of carbon atoms, . Avogadro's number is a dimensionless quantity and has the numerical value of the Avogadro constant given in base units. In contrast, the Avogadro constant has the dimension of reciprocal amount of substance. 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 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). ==History== The Avogadro constant is named after the early 19th-century Italian scientist Amedeo Avogadro, 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.〔 (English translation ).〕 The French physicist Jean Perrin in 1909 proposed naming the constant in honor of Avogadro.〔 (Extract in English, translation by Frederick Soddy ).〕 Perrin won the 1926 Nobel Prize in Physics, largely for his work in determining the Avogadro constant by several different methods.〔Oseen, C.W. (December 10, 1926). ''(Presentation Speech for the 1926 Nobel Prize in Physics )''.〕 The value of the Avogadro constant was first indicated by Johann Josef Loschmidt, who in 1865 estimated the average diameter of the molecules in air by a method that is equivalent to calculating the number of particles in a given volume of gas.〔 (English translation ).〕 This latter value, the number density of particles in an ideal gas, is now called the Loschmidt constant in his honor, and is related to the Avogadro constant, ''N''A, by : where ''p''0 is the pressure, ''R'' is the gas constant and ''T''0 is the absolute temperature. The connection with Loschmidt is the root of the symbol ''L'' sometimes used for the Avogadro constant, and German-language literature may refer to both constants by the same name, distinguished only by the units of measurement. Accurate determinations of Avogadro's number require the measurement of a single quantity on both the atomic and macroscopic scales using the same unit of measurement. This became possible for the first time when American physicist Robert Millikan measured the charge on an electron in 1910. The electric charge per mole of electrons is a constant called the Faraday constant and had been known since 1834 when Michael Faraday published his works on electrolysis. By dividing the charge on a mole of electrons by the charge on a single electron the value of Avogadro's number is obtained.〔(NIST Introduction to physical constants )〕 Since 1910, newer calculations have more accurately determined the values for the Faraday constant and the elementary charge. (''See below'') Perrin originally proposed the name Avogadro's number (''N'') to refer to the number of molecules in one gram-molecule of oxygen (exactly of oxygen, according to the definitions of the period),〔 and this term is still widely used, especially in introductory works. The change in name to ''Avogadro constant'' (''N''A) came with the introduction of the mole as a base unit in the International System of Units (SI) in 1971,〔Resolution 3, 14th General Conference on Weights and Measures (CGPM), 1971.〕 which recognized amount of substance as an independent dimension of measurement. With this recognition, the Avogadro constant was no longer a pure number, but had a unit of measurement, the reciprocal mole (mol−1).〔 While it is rare to use units of amount of substance other than the mole, the Avogadro constant can also be expressed in units such as the pound mole (lb-mol) and the ounce mole (oz-mol). :''N'' = = 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Avogadro constant」の詳細全文を読む スポンサード リンク
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