Amedeo Avogadro

"Avogadro" redirects here. For the Bishop, see Albert Avogadro.

Portrait of Amedeo Avogadro

Lorenzo Romano Amedeo Carlo Avogadro, Count of Quaregna and Cerreto (August 9, 1776–July 9, 1856) was an Italian chemist who provided the solution to important problems in chemistry by postulating that equal volumes of gas at the same temperature and pressure contain equal numbers of molecules. The name "Avogadro's Number" is applied to the number of molecules in a liter of gas at standard temperature and pressure.

Biography

Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e di Cerreto was born in Turin, the son of Cavaliere Philippo Avogadro and Anna Vercellone di Biella. His father descended from an ancient family with a long history in the legal profession.

In 1789, Avogadro received a degree in philosophy, and a baccalaureate in law in 1792. He was awarded a doctorate in ecclesiastical law at the early age of 20. He then established a legal practice which he kept until about 1800, when he began researches in physics. In 1809, he won an appointment as professor of physics at the Royal College Academy at Vercelli.

He submitted his first paper with his brother, Felice, on electricity to the Academy of Sciences in Turin in 1803. In 1804, he was elected a corresponding member of that body.

The memoir for which he is best known, and in which he postulated his important hypothesis, that equal volumes of gas are composed of equal numbers of molecules, was published in 1811. He continued to improve on the exposition of his theory in additional memoirs.

In 1820, Victor Emanuel I created a chair for mathematical physics at the University of Turin, and Avogadro was appointed to the position, which he held until 1822 when it was dissolved due to the political ferment of the time.

Avogadro, however, whose accomplishments had won him respect beyond his political activity, was granted the title of professor emeritus, for which he received an annual salary of 600 lire.

In 1832, the chair was reinstituted, but was occupied in its first two years by the famous mathematician Augustin-Louis Cauchy. In the third year of its new life, the position was given to Avogadro, who held it until 1850, when upon his retirement, it was occupied by his student, Felice Chio.

In 1840 he attended an important scientific congress in Turin, but failed to receive significant recognition.

Avogadro and his wife, Donna Felicita Mazzi, had six sons. One became a general in the Italian Army. Another was president of the Court of Appeal. Avogadro held many public positions dealing with scientific matters, including national statistics, weather and standards of measurement. He became a member of the Superior Council on Public Instruction in 1848. In 1853 Avogadro submitted a final paper to the Turin Academy of Sciences on the behavior of gases subjected to different degrees of compression.

Avogadro died in Turin in 1856.

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, N_A. It is approximately 6.0221415 × 10²³.

Loschmidt first calculated the value of Avogadro's number, now called Avogadro's constant, which is still sometimes referred to as the Loschmidt number in German-language countries (Loschmidt constant now has another meaning). Avogadro's number is commonly used to compute the results of chemical reactions. It allows chemists to determine the exact amounts of substances produced in a given reaction.

Accomplishments

During his 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 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 Essay on a manner of determining the relative masses of the elementary molecules of bodies, and the proportions in which they enter into combination.

Avogadro developed this hypothesis after Joseph Louis Gay-Lussac published in 1808 his law on volumes (and combining gases). It simply states that when the volumes of gases that enter into combination to form a third gas, that the volumes are in simple integral proportions to one another. The greatest difficulty Avogadro had to resolve was the huge confusion at that time regarding atoms and molecules – one of the 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.

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.

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.

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

Response to the theory

The scientific community was well aware of Avogadro's hypothesis. André-Marie Ampère too was able three years later to achieve the same result by another method, but neither of their hypotheses gained rapid acceptance.

Only with studies by Gerhardt, Laurent and Williamson on organic chemistry, was it possible to demonstrate that Avogadro's law was indispensable to explain Gay-Lussac's law.

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.

See also

• Avogadro's constant

References

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