I formed an idea of the inner structure of the atom which, although it contained certain crudities, I have ever since regarded as representing essentially the arrangement of electrons in the atom.
Thermodynamics and the Free Energies of Chemical Substances, Gilbert N. Lewis (1923)
[Of the cubical model] A very crude method of representing certain known facts about chemical reactions. A mode of represent not an explanation.
Lecture notes, Theodore Richards (1902)
‘Atom’ had meant, since the time of Democritus, the smallest indivisible portion of matter. The piece that could not be cut into smaller pieces. That an atom was indivisible was the view held until approximately the beginning of the 20th century.
John Dalton’s early 19th-century atomic theory, improving on Democritus, and derived from experiment, said that all atoms of a given element are identical in mass and properties; that atoms combine in simple whole-number ratios to form compounds; and that a chemical reaction is a rearrangement of atoms.
We now call this the billiard ball model. Atoms, and the compound elements resulting from their combination, were represented like so:
At the beginning of the 20th-century, after discovering the electron, but before the nucleus was known, J. J. Thomson proposed the plum pudding model of the atom. The atom was now divisible. It had smaller components. The negatively charged particles were like plums, floating in a positive pudding.
I have been telling the straightforward story of the atom here. The one that progresses in a linear way, from Democritus, to our knowledge today: Democritus; Dalton; Thomson; Rutherford; Bohr; Schrodinger. The atom is like a billiard ball; it is like a plum pudding; it is like a planetary system; it is a probability cloud.
This story omits Hantaro Nagaoka’s idea, after Thomson but before Rutherford, that electrons are like the rings of Saturn, which would add a nonlinear back-and-forth on the question of whether electrons are particles. It omits ideas influenced by Darwin, not astronomy, that the current atoms had evolved from a more primitive state. It omits now-forgotten ideas about amirs, dynamids, and pantogens. It omits the popular vortex model of the atom from Lord Kelvin, mathematically developed by Karl Pearson. It skips Enlightenment-era thinkers who conceived of the atom as force, not matter, and 19th-century scientists who considered the atom an absurdity. It skips past the anti-atomist views of the earlier Scholastics, which is a rather considerable intellectual history to omit. From Democritus to Dalton, with nothing in between, is certainly a large gap.
But we love the story of steady scientific progress. We love the story of an ever-forward evolution of knowledge, always adjusting to new evidence, from a more primitive state. We love the story of a linear development in models of the atom, for being a great example of how science works.
I love this story. It is a story of how ideas changed about the nature of the atom. These are the notes (and diagrams) I use when I teach the atomic nature of matter to non-science majors. The best thing about this story is that it is a great example of science. Science (or scientists) build a model. If new evidence comes along, the model gets changed.
The Development of the Atomic Model, Rhett Allain
In 1916, Gilbert Lewis published a structure of the atom. Most tellings of the development of the atomic model will omit his version. It is like a square peg in a round hole. It does not fit. All the other models are spherical. In the Lewis model, atoms are like little cubes.
We cannot see inside atoms, of course. But the cubical atom nicely represents some of the known properties. The geometry of eight vertices of a cube fits with the valence bond theory of electrons, which works at multiples of eight.
The position of the electrons, at the vertices of the cubical shells, shows what simple whole-number ratios of atoms are possible as compounds, and the stability of particular combinations, and that compounds can have ionization levels.
From the 1916 paper, a hundred years later, we still teach Lewis dot notation. The crude but useful method of representing certain known facts about chemical reactions.
There is a contradiction or an irony here.
The idea of atoms being tiny little cubes has been discarded. A model of the atom having square edges is as anachronistic as the plum pudding one. We would conceive of atoms as being tiny little spheres. But the model of the cubical atom, though omitted from the standard telling of atomic models, helped us get through chemistry class.
The 2017 March for Science, in Washington D.C, took as symbol a representation evoking the Rutherford model of the atom. Rutherford’s model adjusted for the presence of a nucleus, which was demonstrated by his gold foil experiments of 1908-1913. In the plum pudding model of the atom, particles beamed at a gold foil should pass straight through. The deflections in all directions actually observed by experiment, gave away the presence of a nucleus.
There is some contradiction or irony here.
The symbol has been popular to express the idea of progress and of futurism, long after the model itself was outdated. Using the 1911 Rutherford model of the atom, as a symbol for a 2017 march for science, is a bit like using as a symbol for a march for progress, the T-Model Ford.
Everyone loves it, though.
Atomic Chef holding a futurist plum pudding in his palm, to express how scientific his peanut butter is, would not work so well. Holiday treats do not seem very scientific.
Holding a pair of cubes that represent a chemical bond, would not work so well either, though the cubes would probably teach better chemistry. A weakness in the Rutherford model is that it cannot represent such things as peanut oil being hydrogenated, or cyclamate holding together.
In college, [Linus] Pauling learned and taught the most current and widely accepted model of chemical bonding—the hook and eye model—a name borrowed from the clothing fasteners used at the time. This model proposed that chemical bonds form when the hook of one atom connects with the eye of another atom. Different atoms had different numbers of hooks and eyes, thus dictating the number of bonds that an atom could form. For Pauling, the hook and eye model raised more questions than it answered.
Linus Pauling: American Hero, Sarah Vos
G. N. Lewis' 1916 paper introduced the theory of the shared electron pair chemical bond and revolutionized chemistry.
G. N. Lewis and the Chemical Bond, Linus Pauling
The billiard ball model actually had hooks and eyes, under Democritus and Dalton alike.
We cannot see inside the atom. We want some representation to help us conceive of how they work. For Democritus, atoms joining by hooks and eyes explained how solids held together. For Dalton, additionally that atoms combine in simple whole number ratios.
Gilbert N. Lewis revolutionized chemistry, by explaining chemical bonds via electron pairing. This eliminated the need for hook and eye explanations. Linus Pauling improved on the Lewis model, by unifying explanations of ionization and bonds. We cannot do chemistry without this path in the development of ideas.
The omission of the Lewis cubical atom from so many tellings of the history of atomic models is curious. Omitting the model in favor of a single-path progression of ideas makes for an example of how science did not work.