Some Notes on Avogadro's Number, 6.023 x 1023

Chemists use Avogadro's number every day.  It is a very valuable number for a chemist to know how to use, and use properly. Where did Avogadro's number come from?   Did Avogadro himself do all the calculations?  Was it just arbitrarily made up?   How can it be measured?  Some possible answers follow.

Amadeo  Avogadro (1776-1856) was the author of Avogadro's Hypothesis in 1811, which, together with Gay-Lussac's Law of Combining Volumes, was used by Stanislao Cannizzaro to elegantly remove all doubt about the establishment of the atomic weight scale at the Karlsruhe Conference of 1860.

The name "Avogadro's Number" is surely just an honorary name attached to the calculated value of the number of atoms, molecules, etc. in a gram molecular weight of any chemical substance. Of course if we used some other mass unit for the mole such as "pound mole", the "number" would be different. The first person to have calculated the number of molecules in any mass of substance seems to have been Josef Loschmidt, (1821-1895), an Austrian high school teacher, who in 1865, using the new Kinetic Molecular Theory (KMT) calculated the number of molecules in one cubic centimeter of gaseous substance under oridnary conditions of temperature of pressure, to be somewhere around 2.6 x 1019 molecules. This has always been known as the "Loschmidt Number."

I have been searched for some time to determine the first time the term "Avogadro Number" was used.  My best estimate is that it was first used in a 1909 paper by Jean Baptiste Jean Perrin (1870-1942) entitled "Brownian Movement and Molecular Reality."  This paper was translated into English from the French in "Annales De Chimie et de Physique" by Fredric Soddy and is available.  Perrin, was the 1926 Nobel Laureate in Physics for his work on the discontinuous structure of matter, and especially for his discovery of sedimentation equilibrium.  Perrin should be very well known to Dr. Northrup, of our chemistry department here at TTU  who does calculation in molecular dynamics using methods developed by Perrin.   In his paper Perrin says "The invariable number N is a universal constant, which may be appropriately designated "Avogadro's Constant."

In a simple minded way, here is how it might have come about:

Some Links related to this essay:
A Biographical interview with Amadeo Avogadro