Avogadro's law and some modern formulations of it

Abstract

1. In the century that has elapsed since the death of the great Italian scientist Avogadro (1856-1956), the hypothesis that he advanced in 1811 concerning the equality of the numbers of molecules in equal volumes of gases under identical external conditions has become the basis of modern atomic and molecular theory; it can be regarded not as a hypothesis, but as a law of nature. 2. The field of application of this important generalization has been found to be considerably wider than its author could have expected. Thus, in the course of time the following new formulations have been given: under identical conditions a) equal volumes of solutions contain equal numbers of solute molecules (Avogadro-van't Hoff law), b) equal volumes of solutions contain equal number of ions of strong electrolytes (Avogadro-Arrhenius law). In its application to gases the strict formulation of the law is: equal volumes of gases at the same temperature and at equal infinitesimally low pressures contain equal numbers of molecules (Avogadro-Berthelot law). 3. The analogy between a molecular gas and an electron gas in the volume of an ion of ideal spherical form (an eight-electron ion of inert-gas configuration) makes it possible to give a new formulation to Avogadro's law which is supported by the available experimental data: under identical conditions two ions of equal volumes contain equal numbers of electrons.