The microscopic world of particles produces problems of scale in terms of their almost infintesimally small size and the enormous numbers of them.

This problem was dealt with specifically by Amedeo Avogadro, who developed the work of French chemist Joseph Louis Guy-Lussac (1778-1850) with the 1811 publication of his hypothesis, and the idea that gases are made up of atoms or combinations of atoms (molecules) and can be quantified. Although his work was largely ignored during his lifetime, by the 1880s it was universally accepted, thanks to Stanislao Cannizzaro, who created a table of atomic weights based on Avogadro's work.

This section deals with this quantification and its utility to modern chemists.