Chances are, you’re not like me: you cringe when someone says the word “chemistry.” Your memory of it vaguely harkens back to an undergrad class whose knowledge you flushed once you finished the final. That’s okay — my goal for this series is to change that paradigm, make it understandable to all, and provide some entertainment.
There are two very important things to remember about John Dalton and the beginning of atomic theory:
1) Dalton had no direct physical evidence of atoms.
Zip. Zilch. Nada. Due to this hard to ignore fact, his theory was relegated to chemistry “meta” in some circles of researchers. He did have an abundance of indirect evidence, though, and his theory fit well with the previously established framework.
2) Dalton’s atomic weight system was relative and arbitrary.
Dalton himself recognized these two pieces of the theory could be troublesome. His weights were based off of the fact that since hydrogen was the lightest gas, it should be the benchmark and that all other weights would be based off setting hydrogen’s atomic weight to one.
While he published, presented, and argued his theory until he was blue in the face, he needed to win over minds — he needed support. To accomplish this, other chemists need to incorporate the theory into their work and expand its utility. This article is about two gentlemen who basically acted as atomic theory’s jock strap. (See what I did there? “Jock strap” and “support”?)
The first unsuspecting Dalton wingman was Joseph Louis Gay-Lussac (1778–1850, we’re just going to call him “JGL”). As the son of the king’s lawyer, JGL was one of those well-off scientists we’ve read about a few times now. He studied under Berthollet and Fourcroy, both of whom were students of Lavoisier, so JGL was drinking almost directly from the fountain of greatness. Interestingly, after his forced sabbatical, Berthollet returned to a city south of Paris called Arcueil where he basically started the French version of the Royal Society. Many famous chemists ended up spending time in Arcueil, which lead to the development and publication of its own peer-reviewed scientific journal. It was this journal that contained JGL’s support of atomic theory although neither he nor Dalton realized it at the time.
The Law of Combining Volumes
JGL, like everyone else at the time, was obsessed with exploding hydrogen in oxygen. I mean, who can blame them? It’s a demonstration I’ve done multiple times, and I still get a kick out of seeing everyone’s reaction. JGL’s interest piqued when trying to figure out optimal ratios of hydrogen and oxygen — meaning, he was trying to determine the exact amounts of each that convert each reagent fully to water and leave no hydrogen or oxygen behind. He soon discovered that two volumes of hydrogen combined fully with one volume of oxygen, which resulted in two volumes of water:
2 volumes of H + 1 volume of O → 2 volumes of water
JGL was able to conduct similar experiments and found that gases react with each other strictly in whole number ratios of volumes. The important thing to remember here is that JGL is solely interesting in performing reactions such that all of the starting reagents are consumed completely, and none are left over. Nowadays, we call this situation “stoichiometrically balanced.” In other words, reacting two gases together in ratios of volumes other than whole numbers would ultimately result in leftover starting reagents.
In modern chemistry, chemists will publish their results and then talk about what they mean (answering questions like “so what?” and “what does the data suggest?”). Unfortunately, JGL never really took this step with his results — which kinda sucks because they are very much pointing toward the atomic nature of water and another very important hypothesis.
Avogadro’s Hypothesis: “Equal volumes of gases under the same conditions contain equal number of particles”
While JGL was *this close* to stating the above, it was Amedeo Avogadro (1776–1856, Avogadro’s number is named after him — he didn’t actually discover it) who ultimately published the hypothesis that could reconcile issues raised by Dalton.
Dalton’s problems were derived from the notion that this hypothesis potentially contradicted his idea of the atom. The first concept under fire was the actual composition of water, which Dalton claimed was one atom of water and one atom of oxygen (based on his premise of simplicity). Another chemist, Jons Jakob Berzelius (1779–1848), proposed that water consisted of two atoms of hydrogen and one atom of oxygen (which is correct). Either proposal, however, created problems with Avogadro’s hypothesis.
If you believed Dalton’s proposal about the composition of water, then in order for Avogadro’s hypothesis to be true, then oxygen must split in half to account for the experimental evidence — something that goes against the definition of an atom in atomic theory. If you believed Berzelius, then in order for Avogadro’s hypothesis to be true, two volumes of hydrogen combined with one volume of oxygen should produce one volume of water — again, this went against experimental evidence. The hypothesis that “equal volumes of gas contain equal numbers of particles” was on life support…
Until Avogadro brilliantly defended his hypothesis by thinking outside the box. In 1811, he suggested that these elemental gases were diatomic particles, i.e., in their elemental or natural state, gaseous particles existed as two atoms together: H₂ and O₂. He didn’t use the word “diatomic” (he used the Italian equivalent of “half-molecule,” which I’m sure involved a hand gesture), but he nonetheless was now able to explain the experimental evidence. The simplest way to explain this is reduce the reaction down to individual molecules instead of volumes. Two molecules of diatomic hydrogen yields four hydrogens total for reaction with oxygen. One molecule of diatomic oxygen yields two oxygen atoms for reaction with hydrogen.
Hopefully, you can see where this is going. Four atoms of hydrogen (derived from two diatomic hydrogen particles) react with two atoms of oxygen (derived from one diatomic oxygen particle) resulting in two water molecules (according to Berzelius’ composition of water). The numbers all match up! In addition, Avogadro was able to explain other physical phenomena that had been troubling Dalton as well (e.g., comparative densities). Yay, huge win for atomic theory, #amirite?
Except Avogadro’s hypothesis went completely unnoticed for a half century. That period of 1811–1860 saw an explosion of empirical knowledge, like the discovery of chemical electricity and new elements, but all of it was built on differing theories of atomic weights — remember, Dalton recognized his selection of hydrogen as having an atomic weight of one was arbitrary. Other chemists seeking to improve the system came up with other standards soon after. It was this mass confusion as to what should be accepted as the standard that spurred a conference in 1860 where Avogadro’s hypothesis was brought into the spotlight.
Even though that’s kind of a shitty ending to such a crucial piece of chemical knowledge (Avogadro never knew the impact he had on chemistry), the importance of these two findings is often wrongly understated. The law of combining volumes was cited in numerous chemists’ research, and while Avogadro’s hypothesis was initially thought to be a threat to atomic theory, it ended up solidifying it 50 years later. This begs the question: what happened in those 50 years anyway? We’ll discuss that next time…(hint: a LOT happened.)
Brock, William H. The Chemical Tree: A History of Chemistry. New York: Norton and Co, 2000.
Ihde, Aaron J. The Development of Modern Chemistry. New York: Dover Publications, 1984.