History and progress in the accurate determination of the Avogadro constant

The Avogadro constant, NA, is a fundamental physical constant that relates any quantity at the atomic scale to its corresponding macroscopic scale. Inspired by the kinetic gas theory Avogadro proposed his hypothesis in 1811, in order to describe chemical reactions as an atomic process between atoms or molecules. Starting from his pioneering findings, the determination of this large number has fascinated generations of scientists up to this day. The review of methods aimed at finding a value for NA starts with the calculations made by Loschmidt (1865; NA ≈72×1023 mol-1) who evaluated the number of molecules in a given gas volume, derived from estimates of molecular diameters and the mean free path length. Consideration of Brownian motion led to some more accurate determinations of NA around the beginning of the 20th century (Perrin (1908); NA≈6.7×1023 mol-1). Other methods developed in the following years are based on Millikan's oil drop experiment (1917, NA≈6.064(6)×1023 mol-1), on the counting of alpha particles emitted from radium or uranium (Rutherford (1909); NA≈6.16×1023 mol-1) and on investigations of molecular monolayers on liquids (Nuoy (1924); NA≈6.004×1023 mol-1).