Re: Sources of the value of Avogadro's number

[js_dukelow@ccmail.pnl.gov]

D. Tabor's lovely little book (well, not so little now in the 3rd 
edition) _Gases, Liquids and Solids and other states of matter_, 
Cambridge University Press, 1991, pp. 5-6, describes three early 
experiments to determine the value of Avogadro's number.

In 1909, Rutherford measured the number of disintegrations from a kg of 
radium in a year, measured the volume of He gas released, and used the 
already known datum that a gram-mole of any gas would have a volume of 
22.4 liters at standard temperature and pressure to calculate a value of 
6e23 atoms of He per gram-mole.

Without saying precisely who did the calculation, Tabor notes that the 
facts that
  1)  One Faraday = 96500 Coulombs will convert liberate 1 gram of 
hydrogen gas during electrolysis of a dilute acid; thus if 1 gram of 
hydrogen = N atoms, then  96500 Coulombs = N * e , where e is the charge 
on the electron.
  2)  Using the value of e = 1.6e-19 Coulombs from the Millikan oil drop 
experiment in 1909, gives Avogadro's number = 6.03e23 atoms.

As suggested by Max von Laue in 1912, X-ray diffraction can be used to 
calculate the distance between atoms in the cubic crystalline structure 
of NaCl.  Using that distance, the molecular weight of NaCl, the density 
of crystalline NaCl, and the computed volume occupied by a molecule of 
NaCl in the crystal, Tabor computes the value of Avogadro's number as 
6.02e23 atoms.

Best regards.

Jim Dukelow

js_dukelow@pnl.gov

With the usual disclaimers.