In the opening decades of the nineteenth century, chemistry appeared on the verge of splintering into irreconcilable factions. John Dalton’s atomic theory had captured the imagination of Europe, but a confounding paradox threatened its survival. In 1809, the French chemist Joseph Louis Gay-Lussac (1778–1850) measured the combining volumes of reacting gases and discovered an astonishing regularity: gases react in simple, whole-number ratios by volume. Two volumes of hydrogen combined with one volume of oxygen to yield exactly two volumes of water vapor.
Dalton vehemently rejected Gay-Lussac’s results. In Dalton’s mind, if one volume of hydrogen contained $N$ atoms and one volume of oxygen contained $N$ atoms, then $N$ atoms of hydrogen reacting with $N$ atoms of oxygen should produce $N$ compound particles of water ($HO$)—and therefore exactly one volume of steam. For two volumes of steam to emerge, an atom of oxygen would have had to split in half: a physical impossibility, since atoms were by definition indivisible!
Avogadro’s Brilliant Hypothesis (1811)
In 1811, a reclusive Italian count and professor of mathematical physics at Vercelli, Amedeo Avogadro (1776–1856), proposed the conceptual masterstroke that resolved the crisis. Avogadro made a crucial distinction that had eluded Dalton: the distinction between an atom (the ultimate constituent particle of an element) and a molecule (the independent physical particle of a gas, which could consist of multiple atoms bonded together).
Avogadro’s hypothesis stated: Equal volumes of all gases, at the same temperature and pressure, contain an equal number of molecules:
$$V propto n quad (text{at constant } T text{ and } P)$$
Furthermore, Avogadro dared to suggest that the fundamental particles of pure elementary gases were not solitary atoms, but diatomic molecules: hydrogen existed as $H_2$, oxygen as $O_2$, nitrogen as $N_2$. Under Avogadro’s scheme, Gay-Lussac’s volumes made perfect physical sense without cleaving any atoms:
$$2H_{2(g)} + O_{2(g)} longrightarrow 2H_2O_{(g)}$$
Two molecules of diatomic hydrogen reacted with one molecule of diatomic oxygen to yield two molecules of water vapor. No atoms were broken; the two oxygen atoms in the $O_2$ molecule simply partitioned themselves into two separate water molecules!
The 1860 Karlsruhe Congress: Chemistry United
Tragically, Avogadro was ignored for nearly fifty years. The great European masters—Jöns Jacob Berzelius and Dalton—rejected diatomic molecules because they believed identical atoms, possessing identical electrostatic charges, would repel one another. By the late 1850s, the confusion was unbearable: chemists used nineteen different formulas for acetic acid, and textbooks disagreed on whether the atomic weight of oxygen was 8 or 16, and carbon 6 or 12.
To end the anarchy, August Kekulé organized the world’s first international scientific summit: the Karlsruhe Congress in September 1860 in the Grand Duchy of Baden. Over 140 leading chemists from across the globe gathered in contentious debate. As the conference reached a tense stalemate, a fiery Sicilian chemist named Stanislao Cannizzaro (1826–1910) took the stage. Distributing his brilliant pamphlet Sunto di un corso di filosofia chimica, Cannizzaro demonstrated that applying Avogadro’s law systematically swept away every contradiction, yielding unified molecular weights, establishing vapor density as the true test of molecular mass, and proving conclusively that oxygen weighed 16 and water was $H_2O$.
As the delegates read Cannizzaro’s treatise on their train journeys home, the fog lifted. A young Russian chemist in the audience named Dmitri Mendeleev later reflected: “I vividly remember the impression produced by Cannizzaro’s speech… it seemed as if a light had suddenly been shed on a dark room.” The unified atomic weights forged at Karlsruhe provided the exact foundation Mendeleev needed to discover the Periodic Table.
