By Don Lincoln, Fermilab
When the idea of atoms is introduced, it somehow seems mandatory to mention Democritus, often called the author of atomism. It is unclear if an atomic theory of matter actually originated with him, or with his lesser known teacher, Leucippus. At some level, it really doesn’t matter all that much. Either way, this story unfolded about 400 BC, give or take.
Leucippus
The idea of atomism began when Leucippus noted that an oak tree might die and decay back into dirt. A subsequent acorn, planted in that dirt, would grow and look much like the historical tree. From chains of thought like this, he concluded that there must be some sort of fundamental oak-ness that came together, then disassembled during the decay process, and then reassembled with the subsequent tree. When we consider the data and instrumentation they had access to, it’s actually a strikingly impressive intellectual achievement, although it’s quite wrong in detail.
The wrong bits come from some other equally-reasonable ideas. For instance, Democritus thought that there were atoms of oil, which were physically smooth because of the properties of oil. And he thought that atoms of vinegar were sharp, because of their tart taste. These ideas are wrong, of course, but what can we expect 2500 years ago. It’s better than anyone could have done.
John Dalton
The idea of atomism lay dormant for millennia. It wasn’t until about 1800 that further progress was made when British chemist, John Dalton, noticed some mathematical regularities in how chemical compounds were formed.
Interestingly, by Dalton’s time, chemists had already realized that some substances could be built of other substances. Using modern language, they knew of chemical compounds that could be made from a small number of chemical elements. These elements were the familiar hydrogen, oxygen, carbon, gold, and so on. Chemists also found that what they called the elements could not be broken up into other substances. Hence, the elements were very well elemental. They were the building blocks of everything. But nobody really understood what was going on when the elements combined to form compounds.
Dalton used some work by another scientist by the name of Joseph Proust. Proust was a French chemist who noticed that elements combined in fixed proportions. For instance, he combined tin and oxygen and found that they could combine in two different ways. Using modern nomenclature, he found that he could combine 100 grams of tin with 13.5 grams of oxygen and make tin (II) oxide. Using a different process, he could combine 100 grams of tin with 27 grams of oxygen and make a different substance, now called tin dioxide. But those combinations were all he could do. He couldn’t combine 100 grams of tin with, say, 20 grams of oxygen.
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Quantifiable Evidence that Atoms Exist
What Dalton did was to notice that these numbers, and a few other examples can be described as ratios of small numbers. For instance, in the case of tin and oxygen, one example used 13.5 grams of oxygen, while the other used 27 grams. That’s a ratio of two to one. There are lots of other examples, where the ratios of mass of elements were one to three or two to three. Basically, in most cases, the ratios involved two numbers, both of which were near one.
So, that was the first bit of quantifiable evidence that atoms exist. It was a bit indirect, but the reasoning boiled down to this, that the elements come in fixed, discrete quantities. In the case of tin and oxygen, if the oxygen combined according to a certain amount and also precisely double the amount, then that just sounds like a case of the oxygen coming in fixed quantities, like you can take one oxygen or two oxygens, or so on.
It also stands to reason that there is a unit of oxygen and when we make a chemical compound, we take one of them, then two, then three, etc. Those things are, of course, what we now call atoms.
Was Dalton Right?
Of course, this was over 200 years ago and Dalton didn’t get everything right. For instance, he thought that when two elements combined that there was always a form where the two elements each added a single atom to the mix. For instance, Dalton thought that the chemical formula for water was HO, with one atom each. Of course, we now know that water consists of two hydrogen atoms and one oxygen one, making H2O.
It just goes to show us that even the legends of science can get things wrong. It’s an occupational hazard of trying to figure things out. But it reminds us that figuring things out is hard and the process often proceeds in fits and starts, and it can take lots of people and many years to get it right.
Common Questions about the Birth of the Idea of Atomism
Democritus thought that there were atoms of oil, which were physically smooth because of the properties of oil. And he thought that atoms of vinegar were sharp, because of their tart taste.
By John Dalton’s time, chemists had already realized that some substances could be built of other substances. Using modern language, they knew of chemical compounds that could be made from a small number of chemical elements. These elements were the familiar hydrogen, oxygen, carbon, gold, and so on. Chemists also found that what they called the elements could not be broken up into other substances. Hence, the elements were very well elemental.
Joseph Proust was a French chemist who noticed that elements combined in fixed proportions. For instance, he combined tin and oxygen and found that they could combine in two different ways. Using modern nomenclature, he found that he could combine 100 grams of tin with 13.5 grams of oxygen and make tin (II) oxide.
