Quiet Avogadro

Atoms and molecules form the bedrock of chemistry. But in the early days, there was much confusion. Dalton, of Atomic Theory fame, sometimes used the words interchangeably and as a result he thought the formula of water was HO, and that one atom of hydrogen combined with one atom of oxygen to form one atom of water. In 1811, not long after Dalton’s famous treatise on atoms was published, a quiet Italian professor from Turin, Amedeo Avogadro, published his famous treatise on molecules. No famous scientist commented on it. Hardly anyone knew it existed. The quiet Avogadro wasn’t famous nor did he try to be.

 

At the famous 1860 Karlsruhe meeting that brought together the most eminent chemists in the world to solve the confusion wrought by atomic theory and differing weights and measures, Avogadro had already passed away. We might not have known about him, if not for the young firebrand Stanislao Cannizarro who passionately argued that Avogadro’s hypothesis solved all the arguments in a single stroke. The reception was lukewarm, but several esteemed chemists later read Cannizzaro’s pamphlets that were distributed at the conference, and the revolutionary conversion began. Slowly yet surely, Avogadro’s ideas began to percolate through the chemical community.

 

I’ve been reading about the history of chemistry and its early modern founders in Bernard Jaffe’s Crucibles: The Story of Chemistry (From Ancient Alchemy to Nuclear Fusion, 4^th revised edition). The chapter on Avogadro is preceded by one on Dalton and another on Berzelius, famous for their contributions to atomic theory and its systematization respectively. In my first day of an introductory chemistry class, I clearly define the terms atom, molecule, compound (and somewhat vaguely define the term element). I spend most of my time on Dalton’s Theory and use the systematic terminology and symbols of Berzelius. Avogadro gets no mention until a few classes later when we discuss the mole.

 

Reading Jaffe’s book reminded me that I have not been giving Avogadro his due. Once again he gets overlooked. His famous hypothesis is present in chemistry textbooks: “Equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules.” It doesn’t fit well in that first textbook chapter because for the students to understand the full impact would require us to delve into the properties of gases in detail (which we do later in the semester). Ironically, today’s popular G-Chem textbooks sequence the chapters according to the Molecular approach. And I use Avogadro’s clean definition of a molecule on the first day of class. It’s elegant and obvious to the students. We do not know what it was like to experience the confusion of the nineteenth century chemists. I once considered rearranging the material so that Gases would be the running theme throughout the semester, but that never panned out. (I once designed and ran an activity based on Avogadro’s hypothesis for non-science majors classes; I think I only ran it twice.)

 

Avogadro’s name lives on in his eponymous number, which was only established a century after his famous paper. Jean Perrin painstakingly counted tiny particles for weeks on end through a microscope. Not long after, Irving Langmuir proved that the hydrogen molecule contained two hydrogen atoms. Seems so obvious today, but not back then. Avogadro had, in fact, distinguished an integral molecule (of any compound) from a constituent molecule (only containing a single element) which is how we cleanly define a chemical compound as being composed of two or more elements. Jaffe writes that Gay-Lussac, whose observations led to Avogadro’s hypothesis, had “expressed the hope that ‘we are perhaps not far removed from the time when we shall be able to submit the bulk of chemical phenomena to calculation.’ Avogadro’s contribution to chemistry showed the way. It led also to a final agreement as to the true formula for water.” We take it for granted that balancing chemical equations and stoichiometric calculations are fundamental to the science of chemistry, but this was hard-won territory. And it’s thanks to the quiet professor, Amedeo Avogadro, but also his louder champions.