Amedeo Avogadro







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Life of Amedeo Avogadro                                     (back to section Famous Italians)

Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto (Turin August 9, 1776 Cerreto? July 9, 1856) - better known as Amedeo Avogadro -was an Italian scientist born in the Kingdom of Sardinia ad Piedmont, most noted for his contributions to the theory of molarity and molecular weight. The number of molecules in one mole is called Avogadro's number is honor of him, as is Avogadro's law.

His family's business was the law, and Amedeo followed in his father's footsteps earning a doctorate of law in 1796. 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).

He was apparently well liked by his students, who appreciated is impish sense of humor, and quickly settled down into a happy marriage blessed with six sons. In his free time he did a lot of reading and had a complete set of the current scientific journals in his library printed in four different languages.

During this stay in Vercelli he wrote a concise note (memoria) in which he declared the hypothesis of what we now call 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 Lam?herie'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 mani?e de d?erminer les masses relatives des molecules ??entaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons (complete English text here: [1] ( - First page: [2] (

Avogadro's Law implies that the relationship occurring between the weights of same volumes of different gases (at the same temperature and pressure) corresponds to the relationship between respective molecular weights. Hence, relative molecular masses can be calculated from the masses of gas samples.

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 didn't 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.

In 1814 he published M?oire sur les masses relatives des mol?ules des corps simples, ou densit? pr?um?s de leur gaz, et sur la constitution de quelques-uns de leur compos?, pour servir de suite ?l'Essai sur le m?e sujet, publi?dans le Journal de Physique, juillet 1811 ([3] (, about gas densities.

In 1820 he became a professor of Turin's university; In 1821 he published another memoria, Nouvelles consid?ations sur la th?rie des proportions d?ermin?s dans les combinaisons, et sur la d?ermination des masses des mol?ules des corps and little after M?oire sur la mani?e de ramener les compos? organiques aux lois ordinaires des proportions d?ermin?s.

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). 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.

Very little is known about his private life and his political activity; despite his unpleasant aspect (at least as depicted in the rare images found), he was known as a discreet tombeur de femmes although devoted to a sober life and a religious man. He had six children. Several historical studies would confirm that he had sponsored and helped some Sardinian plotters who were organising a revolution in that island, stopped at the very last moment by the concession of Charles Albert's statute. Some doubts however remain, considering the very little amount of evidence.

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.

The scientific society didn't reserve a great attention at his theory, so Avogadro's hypothesis wasn't immediately accepted when announced. Andr?Marie Amp?e too was able three years later to achieve the same result by another method (in his Sur la d?ermination des proportions dans lesquelles les corps se combinent d'apr? le nombre et la disposition respective des mol?ules dont leurs particules int?rantes sont compos?s), but the same indifferent regard was given to his theories as well.

Only with studies by Gerhardt, Laurent and Williamson on organic chemistry, was it possible to demonstrate that Avogadro's law was indispensable to explain why same quantities of molecules, brought to a vapour state, have the same volume.

Unfortunately, in the performance of related experiments, some inorganic substances showed exceptions to the law. The matter was finally concluded by Stanislao Cannizzaro, as announced at Karlsruhe Congress (1860, four years after Avogadro's death), where he explained that these exceptions happened because of molecular dissociations which occurred at certain temperatures, and that Avogadro's law could determine not only molar masses, but as a consequence, atomic masses too.

Clausius, by his kinetic theory on gases, was able to give another confirmation of Avogadro's law. Not long after, in his researches regarding dilute solutions (and the consequent discovery of analogies between the behaviour of solutions and gases), J. H. van 't Hoff added his final consensus for the triumph of the Italian scientist, who since then has been considered the founder of the atomic-molecular theory.

In honor of Avogadro's contributions to the theory of molarity and molecular weights, the number of molecules in one mole was renamed Avogadro's number. Which is approximately 6.02214199 ?1023.

But in his own time, Avogadro's principle was seriously neglected. Historians of science have several theories as to why this should be so, as Avogadro was a respected scientist during his life. One possibility was that a more famous scientist, J. J. Berzelius, was strongly advocating his "dualism" theory which explained compound substances (molecules?) on the basis that one half of the compound had to have a positive charge and the other half a negative charge (to hold the two halves together). It was hard to see how two atoms of oxygen in one of Avogadro's "molecules" could have different charges.

But the real reason is probably more prosaic. In the clannish world of scientific discovery, it pays to be at the center of the action. Avogadro was by this time a professor, and chairman, of physical chemistry at the University of Turin, but in Italy - far away from the major science centers of England, Germany, France or even Sweden. He never got to rub shoulders with the "great ones" of his day, so his ideas did not receive the credit they deserved.

He was a professor until his retirement at the age of 74. He died on July 9th, 1856.

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