Article | REF: K95 V1

Avogadro's number and the mole concept

Author: Mireille DEFRANCESCHI

Publication date: May 10, 2013

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ABSTRACT

The Avogadro's number is a physical invariant which was put forward as a hypothesis in 1811 and was not conceptually defined until 1955. Its determination is still at the core of recent research. Strongly connected to many units of the international system, a large international collaboration is trying to "count" the atoms contained in 1 kg of quasi-perfect spheres made of a silicon crystal highly enriched with 28Si in order to obtain a more precise value of the standard kilogram which will allow in turn for the determination of the Avogadro's constant.

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 INTRODUCTION

Avogadro's number is one of the fundamental constants. It is used by chemists to link the microscopic world of molecules to the macroscopic world of experiments, and by physicists to define the mass of objects from their elementary constituents.

In chemistry, we sometimes speak of large quantities of matter, sometimes of small ones. In practice, a chemist has to express a "chemical equation" in terms of atoms or molecules and convert it into terms of mass. Obviously, it's impossible to count the number of atoms or molecules contained in a sample of matter. Avogadro's number is used to count particles by measuring their mass. Closely linked to Avogadro's number is the notion of the mole, which links the microscopic world of atoms and molecules to the macroscopic world of grams, kilograms, and so on. Knowledge of Avogadro's constant thus enables molar masses to be determined and quantities of chemical species to be sampled.

In atomic and molecular physics, we use the atomic mass constant (symbol m u ) or unified atomic mass unit (symbol u) :

with Avogadro number, giving 1 u = 1.66054 × 10 –27 kg.

Avogadro's number, or Avogadro's constant, , is the number of atoms or molecules (without interaction, at rest and in their fundamental state) per mole of pure substance, i.e. the number of 12 C atoms contained in exactly 12 g of carbon 12. This number allows us to express...

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KEYWORDS

mole   |   Avogadro's number   |   chemistry   |   fondamental physic


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Avogadro's number and the mole concept