It seems easy, looking back, to dismiss some of the fundamental building blocks of atomic theory as antiquated or primitive; after all, we all know electrons exist, further subdivided into quarks and—maybe—even smaller fundamental pieces. But the history of atomic theory shows a progression in steps, not leaps, and was built on the backs of two men: Democritus and Dalton. Read further to discover what these two individuals proposed in their time that came to build our understanding of the universe as we know it.
The first, most primitive version of atomic theory spawned from a challenge. Parmenides, a pre-Socratic philosopher who lived around 500 B.C., asked, “if things are able to change or move, how do they do it without causing something to originate from nothing?”? His conclusion was that any form of change or movement would result in a real object coming out of thin air, and thus any form of change (either of appearance or of state) must be some form of an illusion. Parmenides believed that objects, once they exist, are completely unchangeable.
The emergent theory of being came from Democritus (ca. 460 BC–ca. 390 BC), who tackled Parmenides’s challenge head-on. Democritus was one of one of the first so-called atomists along with his mentor, already well-regarded for his philosophical ideals and reputation as the “laughing philosopher.” His theory of atomism stated that everything is made up of something else, divisible down to the smallest unit: the atom, from the Greek atomos meaning “indivisible.” He postulated that all atoms exist within an infinite void, and like Parmenides, divided everything into two categories: “being vs. non-being.” Unlike Parmenides, however, he postulated that “non-things” exist just as much as “things” do, with atoms and the void equally real, as opposed to the previous belief that emptiness was simply a lack of substance, with no defining properties of its own.
Democritus classified atoms as the following: infinite in number, solid in makeup, of different shapes and sizes, and attached to one another with hooks, loops, and barbs, similar to Velcro. He believed that the shape and size of an atom determined its function.
Once he divided void from material, he addressed the other prong of Parmenides’s problem: change. His division of atoms from emptiness simplified the question: change is simply a relocation of atoms. Rather than a change of state, all change is a change of location or of orientation—essentially, atoms are immutable, but the things they make up, called kosmoi or “worlds” regardless of size, will disappear over time. He postulated that the important traits of materials that we can see and measure (color, temperature, taste, texture) are not, in fact, a part of the atom, but instead are a perception of how atoms are combined.
Overall, Democritus got several things right. Firstly, atoms exist. Furthermore, they exist in a void, and connect to each other in some way to form other, more complex objects, which we perceive based on the ways that atoms combine to make new materials. However, he simply didn’t have enough scientific knowledge to know that atoms were not solid, nor to find that we perceive materials based on their chemical make-ups, rather than the shapes of the atoms they are made of.
It took nearly 20 centuries for any further advancements in atomic theory. John Dalton (1766–1844), meteorologist and chemist, began a chain of step-by-step modifications to atomic theory that would span centuries. He used his knowledge of meteorology and the atmosphere to build his theory of atomism.
By this time, the periodic table of the elements existed, but although scientists knew there were problems with its organization, they didn't know why. (It would take until Henry Moseley published the second of his papers in 1913 for the periodic table to be reorganized, in no small part based on discoveries by Dalton.) Dalton, nevertheless, found that the atmosphere was made up of different materials—hydrogen, oxygen, and nitrogen, primarily—that all had different weights by studying the motion of air masses in the atmosphere. Each different gasses’ pressure pushing down on the earth was different, and the total pressure was the sum of all of the individual pressures.
This discovery later came to be known as the law of partial pressures. But this discovery confirmed something Democritus hadn’t been able to confirm: different substances are made of different kinds of atoms, and different kinds of atoms have different weights and, in his word, “complexities.” Dalton's discovery both supported the method that the periodic table was currently sorted by—weight—and allowed him to attempt to calculate the chemical make-up of various substances. Most of his attempts were incorrect, but his discovery that different atoms had different weights, and that different substances are made of different kinds of atoms, kick-started the progression of atomic theory.
For more about John Dalton and two important volumes of his in NBLA's collection, see this Ex Libris Universum blog post by Sally Newcomb.
For a further, comprehensive summary of more modern developments in atomic theory, see the CHP Teaching Guide covering the Evolution of Atomic Theory.
“The problem of Parmenides (and its possible solutions).” Tutor Hunt. tutorhunt.com/resource/25715/
Berryman, S. “Democritus.” Stanford Encyclopedia of Philosophy, 7 January 2023, plato.stanford.edu/archives/spr2023/entries/democritus/
“John Dalton.” Science History Institute, 19 May 2023, sciencehistory.org/education/scientific-biographies/john-dalton/
Moseley, Henry. “The High-Frequency Spectra of the Elements.” Philosophical Magazine, vol. 27, 1914. pp. 712-713.
Levine, J. & Summers, M.. (2008). Sulfur Dioxide and the Production of Sulfuric Acid on Present-Day and Early Mars: Implications for the Lack of Detected Carbonates on the Surface. LPI Contributions.