If you're anything like me, when you first taught chemistry topics, you'll have wondered why the mole is named after a small fluffy mammal and how anyone could ever count 6.022x10²³ of anything. You see, the chemistry courses I've taken over the years have been much more interested in teaching the use of these concepts than exploring their origins in any depth, so I've spent a fair while digging into them. This series of articles will examine the history behind the chemistry. So to speak.
Joseph Louis Gay-Lussac (1778-1850)
I'll admit to taking a bit of a liberty with the title of this post. Joseph Gay-Lussac was an enthusiastic hot-air balloonist, but he was an ingenious chemist and physicist first. However, it was his fascination with hot air balloons that focused his studies on the behaviour of gases. During their experiments into gases, he and Alexander von Humboldt (Prussian polymath and adventurer extraordinaire) observed that volumes of hydrogen gas reacted with volumes of oxygen gas in a 2:1 ratio and that the volume of water vapour produced was equal to the volume of hydrogen gas used. Nowadays, we recognise this reaction as 2H₂ + O₂ → 2H₂O. They also noted that volumes of carbon monoxide would react with volumes of oxygen in a 2:1 ratio, producing the same volume of carbon dioxide as carbon monoxide used. Again, we now recognise this reaction as 2CO + O₂ → 2CO₂. They published their findings in 1805, providing the spark of inspiration for Avogadro's two revolutionary hypotheses.
Amedeo Avogadro (1776-1856)
Amedeo Avogadro, an Italian teacher and chemist, took Dalton's ideas on the chemistry and atoms and combined them with Gay-Lussac and von Humbolt's observations. To summarise Dalton's ideas quickly, he theorised that:
Elements are made of atoms (tiny particles).
Atoms of an element are identical in size and mass; atoms of different elements will not have different sizes and masses.
Elements can combine in simple ratios to form compounds.
Chemical reactions involve the rearrangement of atoms, rather than their creation or destruction.
John Dalton (1766-1864)
Avogadro hypothesised that equal gas volumes (at the same temperature and pressure) contained equal numbers of particles. This, as we now know, is true. He is assumed that gas particles maintained a fixed distance from each other (which isn't). He then took this a step further: if there were the same number of molecules in each volume and two volumes of hydrogen reacted with one volume of oxygen to make two volumes of water, then two molecules of hydrogen must be reacting with one molecule of oxygen to make two molecules of water.
However, this couldn't be a direct combination as 2H + O → H₂O as that didn't fit the volumetric data gathered (two volumes of water vapour were created experimentally, rather than just one as in the equation). This led Avogadro to his second hypothesis: that gas particles were divisible and that oxygen particles were dividing during this reaction. He concluded that gas molecules were diatomic: that oxygen molecules were O₂, hydrogen molecules H₂ and so on.
In 1811, Avogadro submitted his paper 'Essay on a Manner of Determining the Relative Masses of the Elementary Molecules of Bodies and the Proportions by Which They Enter These Combinations' to the rather obscure French scientific publication 'Journal of Physics, Chemistry and Natural History'.
He followed this up with a second paper in 1815 'Note on the Relative Masses of Elementary Molecules, or Suggested Densities of Their Gases, and on the Constituents of Some of Their Compounds, As a Follow-up to the Essay on the Same Subject, Published in the Journal of Physics, July 1811' (gotta love those 19th-century scientific paper titles!)
The result of the publication of Avogadro's papers? Very little, initially. It would be an exaggeration to say that they sank without causing a ripple, but not by much. Ampère (of electrical current fame) published something similar a few years later and it got pretty much the same reception as Avogadro's work. It's been suggested that the world wasn't ready for Avogadro's theories (Dalton's atomic theory had only been around for a few years, after all). Maybe it was also a status issue: Avogadro was just a humble high school teacher after all and he was suggesting some very radical concepts. His suggestion of the existence of diatomic molecules wasn't especially well-received, for one. At that time, the only chemical bonding that was recognised was the attraction between positive and negative particles; Avogadro gave no suggestion as to how two identical hydrogen atoms (or oxygen atoms) could be bonded together (as their charge would, presumably, be the same).
Whatever the issue was, it would be the work of another Italian chemist, Stanislao Cannizzaro, to pick up Avogadro's ideas and develop them further into the foundations of modern chemistry with his own seminal paper in 1860. Avogadro, much to my surprise, never determined the number which would later become known as Avogadro's constant. That would be the work of other scientists (Jean Baptiste Perrin and Robert Millikan).
Stanislao Cannizzaro (1826-1910)
All that, however, will have to be a story for another day!
To be continued...
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