page_title: Avogadro constant
section_title: First measurements
hierarchical_section_title: Avogadro constant / History / First measurements
caption_attribution_description: English: Portrait plaque of Johann Josef Loschmidt on the wall of a building in Vienna (Lacknergasse 79). Taken 2018-02-27 16:41:13 at GPS coordinates 48.227339, 16.333528 (per EXIF). Plaque at Google coordinates 48.227285, 16.333515 per Google Street View. This file is a copy of File:Wien18 Lacknergasse079 2018-02-27 GuentherZ GD Loschmidt 0683 02.jpg (originally 3,456 × 4,608 pixels) rectified with Gimp to remove the perspective distortion and cropped to the plaque edges, by User:Jorge_Stolfi. Please refer to the original file for the EXIF metadata. Note that, since the rectification was merely 2D, the salient parts of the portrait are slighly shifted upwards relative to the plaque's background plane.
context_page_description: The Avogadro constant is the proportionality factor that relates the number of constituent particles in a sample with the amount of substance in that sample. Its SI unit is the reciprocal mole, and it is defined as NA = 6.02214076×10²³ mol⁻¹. It is named after the Italian scientist Amedeo Avogadro.
  The numeric value of the Avogadro constant expressed in reciprocal mole, a dimensionless number, is called the Avogadro number, sometimes denoted N or N₀, which is thus the number of particles that are contained in one mole, exactly 6.02214076×10²³.
  The value of the Avogadro constant was chosen so that the mass of one mole of a chemical compound, in grams, is numerically equal to the average mass of one molecule of the compound, in daltons; one dalton being of the mass of one carbon-12 atom, which is approximately the mass of one nucleon. For example, the average mass of one molecule of water is about 18.0153 daltons, and one mole of water is about 18.0153 grams.
context_section_description: 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 n₀ of particles in an ideal gas, is now called the Loschmidt constant in his honor, and is related to the Avogadro constant, NA, by
  where p₀ is the pressure, R is the gas constant, and T₀ 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, NA should not to be confused with the entirely different Loschmidt constant in the 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.
caption: Josef Loschmidt
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