Amedeo Avogadro

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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 term "Avogadro's number" is applied to the number of carbon atoms in 12 grams of pure carbon. Although his theories received scant acceptance in his lifetime, he devoted his life to the pursuit of science, and his ideas were vindicated soon after his death.


Amedeo Avogadro was born in Turin, the son of Cavaliere Philippo Avogadro and Anna Vercellone di Biella. His father was a descendant of an ancient family with a long history in the legal profession.

Avogadro received a degree in philosophy in 1789, 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 that he kept until about 1800, when he began doing research 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.

In 1808, he published, "Considerations on which the state of non-conducting matter must be, when interposed between two surfaces endued with opposite electricities."

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, the king of Sardinia, created a chair for mathematical physics at the University of Turin. Avogadro was appointed to that position, which he held until 1822, when it was dissolved due to the political ferment of the time. As Avogadro's accomplishments had won him respect beyond his political activity, he was granted the title of professor emeritus, for which he received an annual salary of 600 lire.

In 1832, the chair was re-instituted, 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 Appeals. 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.


During his stay in Vercelli, Avogadro wrote a concise note in which he declared the hypothesis of what is now called Avogadro's law:

The number of integral molecules in any gas is always the same for equal volumes, or always proportional to the volumes (Avogadro, 1811).

This memoria he sent to a French scientific journal and it was published in the edition of July 14, 1811, under 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."

It had already been established that if an element forms more than one compound with another element (such as oxygen combining with carbon to form carbon monoxide and carbon dioxide), then the weight of the second element being the same, the weights of the first element that combine with it are in simple integral proportions to each other. This formed the basis of John Dalton's atomic theory.

Avogadro developed his hypothesis to explain Joseph Louis Gay-Lussac's findings that when two gases enter into chemical combination to form a third substance, the volumes of the two gases are in simple integral proportions to one another, such as 1:1, 1:2, or 3:2. If the two gases produce a third gas, that gas is also in simple proportion by volume to the other two.

A good example is water. One volume of oxygen combines with two volumes of hydrogen to form two volumes of gaseous water vapor. According to Avogadro's hypothesis, the two volumes of hydrogen contain twice as many molecules as the one volume of oxygen. This means that two hydrogen molecules combine with one molecule of oxygen to produce two molecules of water vapor. How a single molecule of oxygen could result in two molecules of water, both of which contained oxygen, appeared to be a stumbling block to Avogadro's theory. He solved this by assuming that a molecule of oxygen has at least two atoms of oxygen, one each going to form the two molecules of water vapor.

Said Avogadro:

We suppose, namely, that the constituent molecules of any simple gas whatever … are not formed of a solitary elementary molecule (atom), but are made up of a certain number of these molecules (atoms) united by attraction to form a single one (Avogadro 1811).

This bold hypothesis assumed that there could be an attractive force between two atoms of the same substance to form a molecule, which was at odds with theories of the time that posited electrical forces to hold atoms of unlike charge together, and predicted a repulsive action between two atoms of the same kind.

Avogadro did not actually use the word "atom." He considered that there were three kinds of "molecules," including an "elementary molecule" (corresponding to a modern "atom").

Avogadro published several more papers, one in 1814, and two others in 1821, dealing with the combining weights of chemical compounds.

In 1841, he completed a four-volume work that was in part devoted to the molecular composition of bodies.

Avogadro's number

Avogadro did not attempt to calculate the actual numbers of molecules in equal volumes of gases. This task was first accomplished by the physicist Joseph Loschmidt. Loschmidt used James Clerk Maxwell's calculation, in 1860, of the mean free path of a molecule, that is, the average distance that a molecule moves before it collides with another molecule. In 1865, Loschmidt combined this figure with the difference in volumes between air in its liquid and gaseous states, and arrived at an estimate of the number of molecules in a cubic centimeter of air, often known as Loschmidt's number.

The name "Avogadro's number" for the number of carbon atoms in 12 grams of pure carbon (called a "mole" of carbon, or gram molecular weight) was a twentieth century creation. The scientist Jean Baptiste Perrin is believed to have been the first to use the name "Avogadro's number" in 1909. The best measurements for this number puts it at about 6.0221415 × 1023.

Response to the theory

The scientific community was well aware of Avogadro's hypothesis. André-Marie Ampère reached the same conclusion three years after Avogadro, reasoning that the expansion coefficient of gases under varying pressures are identical for all gases, and this could only be accounted for by each gas consisting of an equal number of particles. Yet, because of the prevailing theories of intermolecular forces and a general confusion over the meaning of a molecule and an atom, Avogadro's hypothesis was adopted by only a small minority of chemists in the several decades after he suggested it.

Studies in organic chemistry by Charles Frédéric Gerhardt, Auguste Laurent, and A.W. Williamson showed 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 in 1860, four years after Avogadro's death. Cannizzaro explained that these exceptions happened because of molecular dissociations at certain temperatures, and that Avogadro's law could determine not only molar masses, but also, as a consequence, atomic masses.

Rudolf Clausius, by his kinetic theory of gases, was able to give further confirmation of Avogadro's law. Not long after, in his researches regarding dilute solutions (and the consequent discovery of analogies between the behaviors of solutions and gases), J. H. van't Hoff added his final consensus for the triumph of Avogadro's hypothesis.

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