On the Birth of Modern Chemistry: I—The Atomic–Molecular Theory from Dalton to Avogadro

The purpose of this paper is to review the basic steps in the development of the atomic–molecular theory with the support of the original documentation. The most important contributions are examined, from the first intimation of the theory by Dalton to the Avogadro hypothesis, intended to reconcile empirical relations observed by Gay-Lussac in gaseous reactions with the atomistic ideas. The genesis and the structure of Dalton’s theory are described, and attention is focused on the rules of the greatest simplicity by means of which the relative weights and sizes of elementary and compounds atoms were determined. The first striking achievements are outlined. A peculiar trait of the Daltonian principles is represented by the rejection of a simple relation between specific gravities of elastic fluids and weights of the ultimate particles. On the contrary, the fixed (and small) ratios of reacting volumes in gaseous reactions, a result discovered by Gay-Lussac, suggest that particles combine according to simple numbers. The equal volume/equal number of particles hypothesis was a substitute better than Dalton’s rules for the determination of the atomic weights and led to the prediction of elementary polyatomic molecules.