Sunday, May 30, 2010

Lewis Structures: What Is Essential Is Invisible

After physicists discovered and defined the basic properties of the naked electron, the next big question was how to describe and understand them in the context of atoms and molecules, thus encroaching the natural domain of chemistry.

G.N. Lewis invented Lewis structures as a way to describe and understand how electrons surround atoms and also how they hold molecules together.  He did this in a non-mathematical, pictorial way in the early 20th century before the birth and subsequent ascent of quantum mechanics. Lewis depicted atoms and their electrons as cubes which could be joined at their edges, vertices, and faces:

The physicists regarded Lewis's theory as laughably crude, particularly the notion that electrons were fixed at certain positions. Lewis in turn was critical of the physicists' idea that electrons were completely fluid, as for example, in J. J. Thomson's plum-pudding model of the atom, because it seemed incapable of explaining the definitive shapes of molecules, for example, the tetrahedral geometry of carbon in countless organic compounds.

By the mid 1920's Lewis had dropped his 3D cubic portrayal of electronic structure; what survives today is rather like a flat 2D projection of those cubes onto a plane.  Lewis would have drawn hydrogen cyanide (the molecule that may have killed him) as:

"What is essential is invisible to the eye"

The lasting importance of Lewis's theory is that it provided chemists with a way (albeit simplified) of visualizing the electronic structures of atoms and molecules. That is perhaps why it endures.

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