It was in Vericelli, while experimenting with gas densities, that Avogadro noticed something surprising: the combination of two volumes of hydrogen gas with one volume of oxygen gas produced two volumes of water vapor. Given the understanding of gas densities at the time, Avogadro had expected the reaction to produce only one volume of water vapor. That the experiment produced two lead him to surmise that oxygen particles consisted of two atoms he actually used the word "molecule".
In his writings, Avogadro referred to three different types of "molecules:" integral molecules most similar to what scientists call molecules today , constituent molecules those that are part of an element , and elementary molecules similar to what scientists now call atoms.
His study of such elementary particles was highly influential in the field of atomic theory. Avogadro was not alone in his study of gases and molecules. Two other scientists—English chemist John Dalton and French chemist Joseph Gay-Lussac —were also exploring these topics around the same time, and their work had a strong influence on him.
Dalton is best remembered for articulating the basics of atomic theory—that all matter is composed of tiny, indivisible particles called atoms. Gay-Lussac is best remembered for his eponymous gas pressure-temperature law. Avogadro wrote a memoria concise note in which he described the experimental gas law that now bears his name.
Though his discovery is now considered a foundational aspect of chemistry, it did not receive much notice in his time. Some historians believe that Avogadro's work was overlooked because the scientist worked in relative obscurity. Although Avogadro was aware of his contemporaries' discoveries, he did not move in their social circles and he did not begin corresponding with other major scientists until late in his career.
Very few of Avogadro's papers were translated into English and German during his lifetime. Additionally, his ideas were likely neglected because they contradicted those of more famous scientists. In , Avogadro published a memoria about gas densities, and in he became the first chair of mathematical physics at the University of Turin.
As a member of a government commission on weights and measures, he helped introduce the metric system to the Piedmont region of Italy.
The standardization of measurements made it easier for scientists in different regions to understand, compare, and evaluate each other's work. Not much is known about Avogadro's private life. Some historical accounts indicate that Avogadro sponsored and aided a group of people planning a revolution on the island of Sardinia, which was ultimately stopped by the concession of Charles Albert's modern Constitution Statuto Albertino.
Because of his alleged political actions, Avogadro was removed as a professor at the University of Turin. However, doubts remain as to the nature of Avogadro's association with the Sardinians. In any case, increasing acceptance of both revolutionary ideas and Avogadro's work led to his reinstatement at the University of Turin in In , Avogadro retired from the University of Turin at the age of In addition, Avogadro was not part of an active community of chemists: the Italy of his day was far from the centers of chemistry in France, Germany, England, and Sweden, where Berzelius was based.
Avogadro was a native of Turin, where his father, Count Filippo Avogadro, was a lawyer and government leader in the Piedmont Italy was then still divided into independent countries. After obtaining his formal degrees, he took private lessons in mathematics and sciences, including chemistry. For much of his career as a chemist he held the chair of physical chemistry at the University of Turin. Berzelius is best remembered for his experiments that established the law of constant proportions.
Yet, in practice, there was a glaring discrepancy between the expected ratio of volumes in the case of water, and the observed ratios Moreover, the densities of the gaseous products were frequently at odds with those of the reactants. For example, in the case of the oxidation of carbon monoxide to form carbon dioxide:. How could a gas that contained both carbon and oxygen be lighter than oxygen itself? The gravimetric and volumetric information would make sense, Avogadro suggested in a French paper in , if equal volumes of gases under the same physical conditions of temperature and pressure contained the same number of 'integral molecules', ie separable aggregates of at least two, but possibly more, particles.
He was suggesting that both elementary and compound gaseous molecules divide at the moment of their reaction with other elementary and compound molecules to form the observed volumes of the products.
For example, water was formed from a half-molecule of oxygen and two half-molecules of hydrogen. Because the numbers of such aggregated or integral particles were the same in equal volumes, it followed that the ratios of their densities relative to a chosen standard density stood in the same ratio as their relative masses.
In the four papers Avogadro published on the subject between and he suggested how, on the basis of this hypothesis, vapour density measurements might be used to determine molecular weights, assuming that oxygen of molecular weight 16 was at least a dimer.
There was never any implication that the particles that made up integral molecules were physical atoms and Avogadro probably did not believe in the existence of such indivisible entities. He did not therefore, as was assumed by later chemists, distinguish between atoms and molecules.
At the time the concept of two-identical atoms of an element combining to form a stable dimer was inconceivable and, indeed, remained mysterious until the emergence of the notion of electron sharing in the early s.
According to the chemical authorities of the day, Dalton and Jacob Berzelius, like atoms were self-repulsive, because they were surrounded either by mutually repulsive atmospheres of heat caloric or of electricity. As an added twist, Avogadro's model had gas molecules surrounded by atmospheres of caloric, leading him to suppose they must be the same size in all gases. A further difficulty was that Avogadro could only determine the vapour densities of a handful of gases, and his theoretical calculations of the vapour densities of solid elements worked to four or five decimal places, based upon the weight of the element that combined with 16 parts one volume of oxygen, seemed entirely hypothetical.
Finally, readers of his papers were probably confused by his cumbersome usage of the terms molecule either an atom or molecule , integral molecule usually the molecule of a compound , constituent molecule one molecule of the element and elementary molecule one atom.
In fact this molecular vocabulary was unique to French writers at the end of the 18th century and provides historians with the clue to understanding Avogadro's research programme. The geographically isolated Avogadro belonged intellectually to the French research school of Lavoisier's colleague Simon Laplace, the mathematician and astronomer, and Lavoisier's chemistry disciple, Claude Berthollet.
Chemical composition was determined by physical conditions and concentration, since these affected the natural affinities between elements. Although Dalton had built the laws of constant and multiple composition into his atomic theory at the beginning of the 19th century, Berthollet's version of the Law of Mass Action as it became known in the s intrigued Avogadro who spent much of his life trying to determine absolute affinity figures from physical data in support of the French programme.
Gases were of a primary interest to him as they were to Berthollet's pupil Gay-Lussac because, unlike solids and liquids, their affinities were completely saturated. Unfortunately, after a promising start, Berthollet's research programme, which involved the caloric theory as well as Laplace's idea of short range forces of attraction and repulsion, led nowhere and was abandoned by French scientists after about Avogadro, however, kept the faith; consequently his later work failed to enhance his stature among European chemists.
Avogadro remained at Vercelli until when he returned to the University of Turin as its first professor of mathematical physics only to lose it a year later when political upheaval caused the chair to be suspended. During the next 10 years he returned to practice law, but was reappointed to the re-established Turin chair in In he and his wife retired to their family estates at Quaregna.
He died on a visit to Turin years ago on 9 July None of his obituaries mentioned the molecular hypothesis, though they described him as 'religious without intolerance, learned without pedantry, wise without ostentation, a despiser of pomp, without care for riches, not ambitious for honours; ignorant of his own worth and fame, modest, temperate, and lovable'.
Although warmly regarded by the Piedemontese as a man of great learning and modesty, Avogadro had been little known outside the Italian peninsular during his lifetime, despite his French offerings. His career supports the views of sociologists of science who stress that historical, social, political and cultural factors play important roles in the emergence of scientific knowledge.
The result was intellectual isolation and invisibility.
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