Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto was an Italian scientist, most noted for his contributions to the theory of molarity and molecular weight. Avogadro's number (the number of molecules in one mole) is named in his honor, as is Avogadro's law.

Born in a noble ancient family of Piedmont, Avogadro was a brilliant student; he graduated in ecclesiastical law at a very young age (20) and began to practice. However, soon after he dedicated himself to the study of physics and mathematics, his preferred sciences, and in 1809 he started teaching them (then called positive philosophy) at a liceo (high school) in Vercelli (where his family had some properties).

Avogadro developed this hypothesis after Joseph Louis Gay-Lussac had published in 1808 his law on volumes (and combining gases). The greatest difficulty Avogadro had to resolve was the huge confusion at that time regarding atoms and molecules – one of most important contributions of Avogadro's work was clearly distinguishing one from the other, admitting that simple particles too could be composed of molecules, and that these are composed of atoms. For instance, John Dalton did not consider this possibility. Avogadro did not actually use the word "atom" as the words "atom" and "molecule" were used almost without difference. He considered that there were three kinds of "molecules," including an "elementary molecule" (our "atom"). Also, a keener attention was given to the definition of mass, as distinguished from weight.

Avogadro's law:equal volumes of gases, at the same temperature and pressure, contain the same number of molecules;

this memoria he sent to De Lamétherie's Journal de Physique, de Chimie et d'Histoire naturelle and it was published in the edition of July 14, 1811 with the title Essai d'une manière de déterminer les masses relatives des molecules élémentaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons.

In 1814 he published Mémoire sur les masses relatives des molécules des corps simples, ou densités présumées de leur gaz, et sur la constitution de quelques-uns de leur composés, pour servir de suite à l'Essai sur le même sujet, publié dans le Journal de Physique, juillet 1811 ([1]), about gas densities.He considered that there were three kinds of "molecules," including an "elementary molecule" (our "atom"). Also, a keener attention was given to the definition of mass, as distinguished from weight.In honour of Avogadro's contributions to the theory of molarity and molecular weights, the number of molecules in one mole was renamed Avogadro's number

With suspicious enthusiasm, he took part in political revolutionary movements of 1821 (against the king of Sardinia), so two years later he was removed from his position (or, as it was officially declared, the university was very glad to allow this interesting scientist to take a rest from heavy teaching duties, in order to be able to give a better attention to his researches).

In 1820 he became a professor of physics at the University of Turin. In 1821 he published another memoria, Nouvelles considérations sur la théorie des proportions déterminées dans les combinaisons, et sur la détermination des masses des molécules des corps and little after Mémoire sur la manière de ramener les composès organiques aux lois ordinaires des proportions déterminées.

However, over time this political isolation was gradually reduced, since revolutionary ideas were receiving increasing attention from Savoy kings, up to 1848 when Charles Albert granted a modern Constitution (Statuto Albertino). Well before this, following the increasing attention to his works, Avogadro had been recalled at Turin university in 1833, where he taught for another twenty years.

In 1841 he completed and published his work in Fisica dei corpi ponderabili, ossia Trattato della costituzione materiale de' corpi, 4 volumes.

Avogadro held public posts in statistics, meteorology, and weights and measures (he introduced decimal metric system in Piedmont) and was a member of the Royal Superior Council on Public Instruction.