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The idea of the atom as the smallest, indivisible unit of matter has a long history that predates John Dalton by millennia, but his scientifically reasoned theory at the beginning of the 19th century was a ground-breaking development in our understanding of this fundamental element of the physical world.
The Origins of the Atom
The idea of an indivisible unit of matter from which all things are made can be found in texts from both ancient Greece and ancient India, but the atom as we know it really got its start in ancient Greece in the 6th century BCE.
The term atom is derived from the word atomos,coined by the ancient Greek philosopher Leucippus and his student Democritus around the 6th or 5th century BCE. Literally meaning 'uncuttable', Democritus, in particular, spread the idea of the atomos as being infinite in number, eternal, and uncreated physical particles that make up all matter.
The ideas of the early atomists--as Leucippus, Democritus, and Epicurus are sometimes called--essentialized the concept that the only real change in the world was that of place--specifically the change in the state of motion or of rest--and that nothing new was ever created and that nothing extant ever ceased to be.
When a person was born, the atomos from which they were composed changed position to make that person what they were. Growth was simply more atomos changing position to join an already existing collection of atomos. When someone died and their bodies decayed, the atomos simply separated and dispersed, and those atomos could be reconfigured afterwards to form a blade of grass or a river. They were essentially the Carl Sagans of their day, reminding all of us that we are all made of star-stuff.
This ends up being closer to the actual reality of matter than the ideas of Democritus' arch-nemesis, Plato, who conceptualized the world being build out of transcendent triangles and polyhedra which gave rise to one of the four elements--Earth, Wind, Fire, and Water. These elements would then combine to make imperfect, physical copies of abstracted, perfect forms of any given thing.
Democritus' work survived the fall of Rome and rode out the European Middle Ages in the Islamic world. The rediscovery of the atomos in Europe came along thanks to the reintroduction of Aristotle, Plato's pupil who debated the competing ideas of the atomos and Plato's triangles in his own works, and the Roman poet Lucretius, who wrote of Epicurus' atomist ideas, which built on Democritus' ideas from a couple of centuries earlier.
The reintroduction of pagan philosophy got the governing Church authorities quite bent out of shape, though Aristotle at least had the benefit of a monotheistic-ish philosophy that proponents could--and did--argue demonstrated that Aristotle was really talking about the Abrahamic god, he had just never heard of him so didn't know what name to give his Prime Mover.
Epicurus and Democritus, however, had no such defense. The atomos meant that no gods were needed to explain life and death, or how wood burns to smoke and ash, or how water and soil turns into crops. Everything could be explained by a change in the position of the various atomos in relation to one another. The materialist basis for the philosophies of Epicurus and Democritus directly contradicted Church teachings and so the fruits of that philosophy, the atomos, were branded as foolish pagan heresy, making it dangerous to advocate for such a material model.
Still, there was no getting around the fact that the atomos was a really good way to explain natural phenomenon, so the idea of the atomos stuck around, even being taken up by some within the church who argued that nothing in the scriptures precluded God from creating the universe out of atomos. By the time of the Enlightenment, knowledge of the atomos was fairly widespread among the new scientific class but it remained a purely philosophical idea, by and large.
John Dalton's Work on Gases
At the turn of the 19th century, John Dalton was an English chemist, physicist, and meteorologist working as a secretary of the Manchester Literary and Philosophical Society. By 1800, chemistry had undergone one of the most dramatic intellectual revolutions in millennia as scientific rigor began being applied to the ancient study of alchemy, which came to be called the Chemical Revolution of the 18th century.
While the ancient Greek idea that water, air, fire, and earth were the essential elements of all matter was still taken as a given by many at the time, chemists like Antoine Lavoisier laid much of the groundwork for modern chemistry during the 18th century by isolating and identifying some of the most important elements in chemistry, such as hydrogen and oxygen. Still, this scientific understanding of chemistry and the atom at the center of it all was still in its infancy by the time John Dalton inherited it at the start of the 19th century.
The properties of gases were of particular interest to Dalton and much of his most important work revolves around their study. Starting in 1800, Dalton began recording the different pressures of different forms of vapor, which at the time was considered a separate substance from atmospheric air. According toUniverse Today:
[b]ased on his observations of six different liquids, Dalton concluded that the variation of vapor pressure for all liquids was equivalent, for the same variation of temperature, and the same vapor of any given pressure.
He also concluded that all elastic fluids under the same pressure expand equally when heat is applied. Further, he observed that for any given expansion of mercury (i.e. noted rise in temperature using a mercury thermometer), that the corresponding expansion of air is proportionally less, the higher the temperature goes.
This became the basis [of] Dalton’s Law (aka. Dalton’s law of partial pressures), which stated that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.
It was during this work on the properties of these gases that Dalton noticed a peculiar trend. He found that certain gases could only be combined in specific ratios to form certain compounds, even when two different compounds shared an element or elements in common.
Dalton began to deduce that if a compound could only be made with specific proportions of component elements, the only way this could work is if individual units of the component elements were combining discretely in the mixture at a specific ratio to give rise to one compound and not another.
He further concluded that if two elements can produce two or more compounds, the way carbon and oxygen can make both carbon monoxide and carbon dioxide, the ratio of the second element's masses given a fixed mass of the first element would inevitably be reducible to small whole numbers.
Essentially, if adding a certain amount of oxygen to carbon gives you carbon monoxide, getting carbon dioxide requires adding a multiple of the amount of oxygen used to produce the carbon monoxide, which in this example would mean you'd need to add twice as much oxygen to get carbon dioxide as you needed to get carbon monoxide.
Again, the only way this could be the case is if the physical carbon and oxygen substances you were combining were a collection of individual carbon and oxygen units that would individually couple together in specific ratios according to the amount of each element present.
These two insights, combined with laws on the conservation of mass and of definite proportions--discovered by Lavoisier and Joseph Louis Proust, respectively--were the essential link between the ancient Greek atomos of Democritus and modern chemistry. Dalton recognized this history, so he called these elemental units atoms.
Dalton's Atomic Model
Proposing what would come to be known as the Dalton Atomic Model, Dalton described five essential properties of the atom.
First, every element can be reduced to a single, indivisible unit of itself.
Second, every atom of an element is identical to every other atom of that element.
Third, atoms of different elements were distinguishable by their atomic weights.
Fourth, individual atoms of one element combine with individual atoms of another element to form compounds.
Fifth, no atom can ever be destroyed or created in a chemical process, only the arrangement of atoms changes.
While some of these would turn out to not be entirely correct--isotopes of an element, for example, can differ from one another and even have different properties while still being classified as the same element--what Dalton described at the start of the 19th century is pretty close to our understanding of matter on the macro level today.
How Dalton's Atomic Model was Refined
Over the next century, Dalton's Atomic Model would be refined as further experimentation showed that the atom wasn't as neat and tidy as Dalton first proposed. Marie and Pierre Curie discovered that atoms of certain elements released radiation, which they could not do if they were the irreducible material that Dalton described.
Later, it would be found that atoms could have an electromagnetic charge, either positive or negative, which we call ions. These ions indicate that normally-neutral atoms must be made up of a negatively-charged substance directly proportional to a positively-charged substance so that these two charges canceled each other out. Ions could only be explained if this balance was disrupted, which meant that the electromagnetically charged substances of the atom had to be distinct and separable. The atom, then, wasn't as small as it gets.
From there, we got the proton, the neutron, and the electron; the photon and the Planck constant; and Albert Einstein, Niels Bohr, and others unraveling what by now was the heavily revised atomic model of John Dalton and introducing the bizarre world of quantum mechanics. From there, science leaves the orderly and measurable atomic structure, as well as physics, behind--though no word yet on whether Plato was right about those triangles.