August 9, 2026 marks 250 years since the birth of Amedeo Avogadro. The date finds the Italian city of Vercelli and the University of Eastern Piedmont in the midst of an official program dedicated to the scientist, which began in May and is scheduled to continue until November 23. The anniversary also offers an opportunity to revisit one of the most instructive stories in science: Avogadro formulated an idea in 1811 that is now fundamental to chemistry, but its widespread acceptance would only come about half a century later.
The delay cannot be explained simply as a lack of vision on the part of contemporaries. Avogadro's hypothesis required chemists to accept consequences that conflicted with influential theories of the time, appeared in a field still confused about the meaning of atom and molecule, and lacked, in 1811, the set of evidence and methods that would later make it especially useful. When Stanislao Cannizzaro revived this idea in 1858, the scientific scenario was already different.
What Avogadro proposed in 1811
Avogadro started from Joseph Louis Gay-Lussac's results on the simple proportions in which gases participated in reactions. His proposal was that, maintaining the same temperature and pressure conditions, equal volumes of gases would contain the same number of particles — in modern terminology, molecules. From this relationship emerged a powerful tool: comparing gas densities would allow obtaining relative molecular masses.
The boldest point came when this hypothesis was applied to gases formed by elements. To make the experimentally observed proportions work, Avogadro admitted that a particle of a simple substance did not need to correspond to a single atom. It could contain more than one. This is precisely the now familiar possibility of molecules such as H₂ and O₂.
This solution helped to reconcile John Dalton's atomic theory with experimental results on gaseous volumes. But for many early 19th century chemists, it created a greater theoretical problem than the one it was intended to solve.
Why the idea seemed so difficult to accept
One of the barriers was theories about how particles come together. The electrochemical model associated with Jöns Jacob Berzelius conceived of chemical combination in terms of attractions between parts with opposite electrical properties. Within this view, the stable existence of a molecule formed by two equal atoms, such as O₂, was difficult to justify: particles of the same nature should not come together in that way.
It was not, therefore, just a matter of choosing between two formulas. Accepting Avogadro meant changing assumptions about the very constitution of matter.
There was still a second obstacle: the scientific language was not sufficiently stabilized. Terms equivalent to atom, molecule and particle were used in different ways by different authors. Avogadro himself used expressions such as “integral molecule”, “constituent molecule” and “elementary molecule”. The operational distinction between atom and molecule that seems elementary today was still under construction.
This confusion had practical consequences. Without agreement on what exactly constituted a molecule and how many atoms were in it, different systems of atomic weights and different chemical formulas could coexist. By the mid-19th century, divergences of this type had become a central problem for a rapidly growing chemistry, especially in the study of organic compounds.
The hypothesis also needed more mature chemistry
The 1811 proposal did not have the same empirical and operational strength that it would acquire decades later. The number of substances suitable for reliable vapor density determinations was limited, there were apparent exceptions, and there was a lack of a consolidated theory capable of satisfactorily explaining how like atoms could form stable molecules.
Over the following decades, chemistry accumulated many more experimental results. New studies on gases, vapor densities, composition of substances and dissociation have made it possible to reinterpret problems that previously seemed incompatible with the hypothesis. At the same time, theories that made it difficult to accept molecules made up of identical atoms lost influence.
Avogadro also worked relatively far from the main international chemical networks in France, Germany, England and Sweden. This isolation contributed to the low impact, although it does not alone explain the oblivion: his 1811 work was published in the Journal de Physique and was not simply an unknown text. The problem was that the community still didn't have enough reasons to transform it into the organizing principle of all chemistry.
Cannizzaro showed what the hypothesis was for
The turning point occurred with Stanislao Cannizzaro. In 1858, he published Sunto di un corso diphilosophica chimica, a work in which he explicitly took up Avogadro's hypothesis and used it to establish a coherent distinction between atoms and molecules and determine molecular and atomic masses.
This difference is important. Cannizzaro did not simply assert that Avogadro was right. He showed how to use the hypothesis as part of a system capable of organizing results that chemistry had accumulated over nearly fifty years.
Based on the densities of the gases, it was possible to compare molecular masses. Analysis of the composition of these molecules helped, in turn, to identify the minimum amounts of each element that appeared repeatedly in the compounds. The method offered a much more consistent basis for defining atomic weights.
It was in this new context that the old hypothesis stopped seeming just an ingenious speculation and began to function as a tool to solve concrete problems.
Karlsruhe was a catalyst, not a vote on Avogadro
The Karlsruhe Congress, held from September 3 to 5, 1860, brought together 127 chemists and had among its objectives precisely to discuss the confusion surrounding atoms, molecules, atomic weights, formulas and nomenclature. Names like Cannizzaro, Dmitri Mendeleev and Lothar Meyer participated.
It is common to summarize this episode by saying that Karlsruhe established Avogadro's hypothesis. The story is more gradual. The congress did not reach a definitive solution to all the problems discussed. The decisive impact came, in part, from the circulation of copies of Cannizzaro's text among participants, allowing their ideas to be examined after the sessions. In the following years, the Avogadro-based interpretation gained increasing acceptance.
This standardization was relevant for later advances. A more coherent set of atomic weights provided part of the conceptual basis used by researchers such as Mendeleev and Lothar Meyer in developing the periodic classification of elements.
Avogadro, however, did not see this turnaround. He died in 1856, two years before the publication of Cannizzaro's work and four years before the Karlsruhe congress.
Avogadro's law is not Avogadro's number
Further recognition created another association that could cause confusion. Avogadro's contribution was the hypothesis about the relationship between volume and number of molecules in gases; he did not experimentally determine the value that we now call Avogadro's number or constant. IUPAC itself draws attention to this historical distinction.
Today Avogadro's constant, (N_A), is one of the seven defining constants of the International System of Units. Its value is fixed exactly at 6.022 140 76 × 10²³ mol⁻¹. Since the SI redefinition that came into effect in 2019, one mole contains exactly that number of specified elementary entities.
There is a historical irony in this trajectory: an idea that spent decades without consensus ended up linked to one of the constants that today help define the international system of measures itself.
Why 250 years matter in 2026
Avogadro was born in Turin on August 9, 1776. Between 1809 and 1819, he taught physics at the Regio Collegio de Vercelli, during which time he developed the hypothesis published in 1811. In 2026, the University of Eastern Piedmont, the Municipality of Vercelli and several scientific and cultural institutions organized a calendar of celebrations between May and November.
The program includes academic and cultural activities, as well as events planned for the second semester. Among them are a meeting at the Accademia delle Scienze, in Turin, on October 13; Avogadro Day at the scientific high school that bears his name, on October 23; and the official end of the celebrations on November 23rd. As this information belongs to an ongoing agenda, it must be reconfirmed through official channels before any service is released.
Avogadro's 250 years thus serve to remember something greater than a school formula. The history of his hypothesis shows that a good scientific idea does not win just because it is elegant or, in retrospect, correct. It needs to be compatible with available evidence, overcome competing models, gain a sufficiently clear language and, above all, demonstrate that it can organize problems that previously seemed disconnected.
This is what was missing in 1811. And this is what the chemistry of the following decades, culminating in Cannizzaro's work, finally made possible.
