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"Valence and Molecular Structure," Lectures 1 and 2.

"Valence and Molecular Structure," Lectures 1 and 2. 1957.
Produced for the Institutes Program of the National Science Foundation. Robert and Jane Chapin, producers.

Lecture 2, Part 2. (3:15)


Linus Pauling: A very important contribution to structure theory was made in 1874 by the Dutch chemist van’t Hoff and the French chemist, Le Bel independently of one another. This is the idea that the four bonds formed by the carbon atom are not directed out toward the corners of the square in one plane as indicated here on the blackboard, or are not so loose-jointed that they have no well defined direction, but instead are directed toward the four corners of a tetrahedron.

All of our models are built in this way. Here we have the methane molecule with the four bonds shown proceeding toward the four corners of a tetrahedron, a regular tetrahedron. This has been found in recent years by the determination of the structures of crystals and of gas molecules, by the x-ray diffraction method, the electron diffraction method, and various spectroscopic methods, that the angles between single bonds formed by a carbon atom remain in all substances quite close to the tetrahedral angle, the angle for a regular tetrahedron, a hundred nine degrees, twenty-eight minutes.

The tetrahedral carbon atom is a very important part of chemistry. I think that the discovery of the tetrahedral carbon atom was a wonderful thing – it shows the power of man’s mind. The facts were that in 1874, it was known as a result of the work of Pasteur that some substances can form crystals that have either a left-handed appearance or a right-handed appearance.

Van’t Hoff and Le Bel asked how is it possible for substances to be built up of molecules that are right-handed or left-handed? Two different kinds of molecules that are related to one another in the way that the right hand and the left hand are related. They showed that the tetrahedral carbon atom provides the explanation of these facts, that the four bonds of a carbon atom are connected to four different kinds of atoms or groups.

For example, a hydrogen atom, a methyl group CH3, a chlorine atom, a bromine atom. Then, this tetrahedral molecule can be either a right-handed molecule or a left-handed molecule and the right-handed molecule does not become left-handed by any translational or rotational motion in space. Only by breaking the bond and moving it around to the other side can you convert the right-handed molecule into the left-handed molecule. Recent investigations, recent structure determinations, have of course, completely verified this idea of the tetrahedral carbon atom.


Associated: Linus Pauling, Robert Chapin, Jane Chapin, National Science Foundation
Clip ID: 1957v.1-11

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Creator: National Science Foundation
Associated: Linus Pauling, Robert Chapin, Jane Chapin

Date: 1957
Genre: video
ID: 1957v.1
Copyright: More Information

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