## The History of Avogadro's Number: 6.02 x 10²³

##### Question:

What is the historical reference on the origin of Avogadro's number? How was it first determined?

##### Answer:

Avogadro proposed his hypothesis in 1811. At that time there was no data at all on the number of particles in a mole, or an agreement on any atomic weights or the standard. The first measurements which could give an approximate value for Avogadro's number were observations of brownian motion by Robert Brown in 1827.

Cannizarro (1860) used the Avogadro's hypothesis to develop a defensible set of atomic weights based on 1/16 of the atomic weight of oxygen. This was the basis for progressively more accurate estimates for Avogadro's number over the next 100 years. Reasonable values were available in the late 1800's from sedimentation equilibria of colloidal particles. Millikan's oil drop experiment in the early 1900's gave improved accuracy and was cited in most chemistry text books 50 years ago. Text books in 1958 gave Avogadro's number as 6.02 times 10 to the 23rd. The current value is 6.022137 times 10 to the 23rd.

The answer to the above question was supplied by Reed Howard, Department of Chemistry and Biochemistry, Montana State University.

It is difficult to imagine such a large number as Avogadro's number. Some idea of its magnitude is given by the following calculation. Let us suppose that the entire state of Texas, with an area of 262,000 square miles, were covered with a layer of fine sand 50 feet thick, each grain of sand being 1/100 of an inch in diameter. There would then be Avogadro's number of sand particles in this immense sandpile. There would be the same number of molecules of water in one mole of water -- 18g, 1/25 of a pint.

Comparison supplied by Bob Everson, Animal Disease Diagnostic Lab, Purdue University.