Amedeo Avogadro

Italian physicist

• Born: August 9, 1776
• Birthplace: Turin, Kingdom of Sardinia (now in Italy)
• Died: July 9, 1856
• Place of death: Turin, Kingdom of Sardinia (now in Italy)

A pioneer in atomic theory, Avogadro was the first scientist to distinguish between atoms and molecules. Avogadro’s law, a hypothesis that relates the volume of a gas to the number of particles present, greatly advanced the understanding of chemical reactions and resolved many chemical problems.

Early Life

Amedeo Avogadro (AH-vah-GAH-droh) was born in Turin in the Kingdom of Sardinia about 60 miles southwest of Milan in what is now Italy. He was one of four sons born to Count Filippo Avogadro and Anna Maria Vercellone. Count Avogadro was a distinguished lawyer and civil servant who came from a prominent family in the region that had produced many generations of Italian military and civil administrative leaders. The name Avogadro possibly is derived from the Italian word avvocato (barrister).

As a young child, Avogadro likely received his first education at home from the local priests; he later attended secondary schools in Turin. Between 1792 and 1796 he studied law at the University of Turin with the intention of following his father in a legal career. For some years after graduating from law school, he held several government positions. Around 1800 he began to show an interest in natural philosophy, undertook private study of physics and mathematics, and attended physics lectures at the university. His interest in science seems likely to have been stimulated by the recent research on electricity by fellow Italian Alessandro Volta, who came from neighboring Lombardy.

After 1806, Avogadro abandoned his interest in a legal career to concentrate on science and, with one of his brothers, began working on electricity experiments. He was soon appointed as a demonstrator at the Academy of Turin. In 1809 he became professor of natural philosophy at the Royal College of Vercelli. Within one decade, Avogadro was elected as a full member of the Turin Academy of Sciences and one year later was appointed to the first Italian chair of mathematical physics at Turin. His salary was six hundred lire per year.

Life’s Work

Avogadro was a prolific writer and published articles in many areas of the physical sciences throughout his life. His name appears in most modern chemistry and physics textbooks, although he has often been misrepresented as being a chemist because his work had a profound influence on the development of chemical theories. His name is usually associated with two important aspects of chemistry: Avogadro’s law , which describes the relationship between the volume and number of particles of a gas, and the Avogadro number, which represents the number of particles in one mole of a substance. The concept of the mole as a unit for the measurement of atomic particles was unknown in Avogadro’s time, and the term was not introduced until the twentieth century.

In many ways it is remarkable that Avogadro had a successful scientific career. He received no formal training in science and made a dramatic career change when he was thirty years old. The former was not particularly unusual in the eighteenth and nineteenth centuries, as some of the greatest scientists of the period were self-educated, including Humphry Davy and Michael Faraday. Others (for example, Nicolas Lémery and Jakob Berzelius) had received their early training in a related field such as pharmacy or medicine before concentrating on a career in the physical sciences.

It was surprising that Avogadro made the transition from law to science so effortlessly, and it was an obvious testament to his good mind and dedicated spirit of discovery. However, Avogadro was not a good experimentalist and had a poor reputation as such among his colleagues. He preferred to interpret the experimental results of others using a mathematical approach. Much of his work was translated and published, but it generally appeared in obscure journals. In addition, Turin was geographically isolated from the world centers of scientific research, which were generally considered to be in Germany and France. Finally Avogadro was, by nature, modest and reserved, and he never actively sought fame. He never traveled to other countries and rarely corresponded or met with other scientists outside his region. It was not until after his death that the world really comprehended and recognized his contributions to science.

During the early nineteenth century, chemists began serious attempts to understand the nature of matter and chemical reactions. John Dalton measured the mass ratios of elements in compounds and found these ratios to always be simple whole numbers. For the first time, he demonstrated that the elements must exist as discrete units, or atoms. The nature of one particular form of matter, gases, had always been difficult for early scientists to understand. In 1808, Joseph-Louis Gay-Lussac published studies on the combining volumes of gases. He showed that gases always combined in simple whole number ratios. For example, 200 cubic centimeters of hydrogen always combined with 100 cubic centimeters of oxygen to form 200 cubic centimeters of water vapor (a 2:1:2 ratio).

Although such observations suggested that equal volumes of gases contained equal numbers of atoms, Dalton rejected this hypothesis, believing that Gay-Lussac’s experiments were inaccurate. Dalton and others argued that one volume of oxygen gas contained a specific number of oxygen atoms and therefore must produce the same volume of water vapor with an equivalent number of water atoms. It should be remembered that at the time, it was still generally believed that water had a chemical formula of HO and was composed of HO atoms.

Like Dalton, most chemists of the day believed that common gaseous elements such as hydrogen, oxygen, nitrogen, and chlorine existed as individual atoms. Avogadro’s explanation appeared in his 1811 article Essai d’une manière de déterminer les masses relatives des molécules élémentaires des corps et les proportions selon lesquelles elles entrent dans ces combinaisons (essay on a manner of determining the relative masses of the elementary molecules of bodies and the proportions in which they enter into combinations), in which he attempted to explain the inconsistencies with existing theories by assuming that equal volumes of all gases contained equal numbers of molecules rather than atoms, provided conditions of temperature and pressure were kept constant. Avogadro’s hypothesis (also known as the molecular hypothesis) would later become known as Avogadro’s law. During a chemical reaction, therefore, Avogadro proposed that molecules could split into half-molecules (atoms) and combine with other half-molecules to form the observed product compounds.

By contrast, Dalton viewed the combination of two gases such as hydrogen and oxygen as involving individual atoms. It is now known, thanks to Avogadro’s insight, that this reaction involves molecules that are composed of two atoms each (diatomic molecules), which is consistent with Gay-Lussac’s experimental results on combining volumes in which two volumes of hydrogen react with one volume of oxygen to generate two volumes of water vapor. According to Avogadro, each molecule of water must contain one molecule of hydrogen (H₂) and one half-molecule of oxygen (one O atom). It followed from this that the correct chemical formula for water was H₂O and not HO as Dalton and others believed. It should be noted that Avogadro never used the modern system of chemical formulas that are shown in the previous equations. If he had, his theory may have been more understandable and therefore readily accepted sooner.

Avogadro’s hypothesis also explained discrepancies in the measured densities of gases and resulted in more accurate determinations of atomic weights. Water vapor was known to have a lower density than oxygen, but this fact was difficult to explain if the latter existed as single atoms. The occurrence of diatomic oxygen molecules easily explained why oxygen had a greater density than water vapor. In Dalton’s early table of atomic weights, which were a measurement of the relative weights of atoms, hydrogen was assigned a value of 1 and oxygen a value of 7.5. When viewed as diatomic molecules, the value of hydrogen became 2 and oxygen 15. In other words, Dalton’s atomic weights had to be doubled for diatomic molecules. This eventually led to a more accurate table of atomic weights.

Avogadro’s molecular hypothesis was largely ignored during his lifetime. His theory did have the support of a fellow Italian chemist, Stanislao Cannizzaro, who was one of the few who seemed to grasp the significance of Avogadro’s idea, but only after Avogadro’s death. Most scientists, however, failed to distinguish between atoms and molecules, and Avogadro, isolated in Turin and largely unknown in Europe, never witnessed the universal acceptance of his theory. Cannizzaro showed that Avogadro’s theory could be used for determining molecular size and accurate chemical formulas. His enthusiasm for the molecular hypothesis had a profound influence on the German chemist Lothar Meyer. In his 1864 textbook, Meyer employed Avogadro’s hypothesis to develop his ideas on theoretical chemistry. This book had considerable influence on other chemists, who applied Avogadro’s ideas to many other aspects of physical chemistry.

Avogadro held his position as the chair of mathematical physics at Turin from 1820 until 1822, when it was abolished because of regional political turmoil. The position was reestablished in 1832, and Avogadro was reappointed in 1834. He held this post until his retirement in 1850 at the age of seventy-four. He spent the last six years of his life continuing with his scientific studies and died in Turin on July 9, 1856.

Significance

It was not until around 1870 that the term “Avogadro’s law” first appeared in print; by the 1880’s it had received universal recognition. The realization that common elemental gases existed as diatomic units had an enormous influence on obliterating chemical inconsistencies and linking the chemical and physical properties of substances. Accurate density determinations and atomic and molecular weight measurements for gases also became possible, which aided the rapidly developing area of organic chemistry in the nineteenth century.

Once Avogadro’s law was understood, a new era in the development of chemical theories and molecular composition became possible. The Dutch chemist Jacobus van’t Hoff showed that Avogadro’s law could be applied to solutions as well as gases, for which he was awarded the first Nobel Prize in Chemistry in 1901. For chemists, a significant consequence of Avogadro’s law was the realization that one mole of all substances (that is, the atomic or molecular weight of a substance expressed in grams) contains the same number of particles. This quantity, equal to 6.02252 × 10²³, is now known as the Avogadro number in honor of a great scientist who was not recognized in his own lifetime.

Bibliography

Causey, Robert J. “Avogadro’s Hypothesis and the Duhemian Pitfall.” Journal of Chemical Education 48 (June, 1971):365-367. This short article looks at the role of certain chemists in the history of the delayed acceptance of Avogadro’s hypothesis.

Fisher, Nicholas. “Avogadro’s Hypothesis: Why Did the Chemists Ignore It?” Parts 1 and 2. History of Science 20, nos. 2 and 3 (1982): 77-102, 212-231. Fisher examines the reasons why it took so long for Avogadro’s ideas to become widely accepted.

Holmyard, E. J. Makers of Chemistry. London: Oxford University Press, 1962. Section 52 discusses Avogadro’s work in the context of its historical importance. Contains a sample of his handwriting.

Ihde, Aaron J. The Development of Modern Chemistry. New York: Harper & Row, 1964. Ihde gives a good account of the Avogadro story in one of the classic texts on chemical history.

Jaffe, Bernard. Crucibles: The Story of Chemistry. New York: Dover, 1976. This is one of the most delightful and easy to read accounts of the history of chemistry. Chapter 9 is devoted to Avogadro.

Lavere, Trevor H. Transforming Matter: A History of Chemistry from Alchemy to Buckyball. Baltimore: Johns Hopkins University Press, 2001. Chapter 8, “The Rise of Organic Chemistry,” contains information about Avogadro and Avogadro’s hypothesis.

Morselli, Mario. Amedeo Avogadro. Dordrecht, Netherlands: Reidel, 1984. This is the most thorough account of Avogadro’s life. Morselli describes most of his major contributions to science, not just the molecular hypothesis. Includes bibliographies for each chapter.