The Direction of Valence Bonds in Space

In 1874 it was discovered that the four bonds formed by a carbon atom are directed in space toward the four corners of a tetrahedron. This discovery was made through the effort to explain the observed effects of some substances on polarized light, as described in the following paragraphs. 

When a beam of ordinary light is passed through a crystal of calcite, it is split into two beams. Each of these beams is a beam of plane-polarized, light the vibrating electric field of the light lies in one plane for one of the beams and in the plane at right angles to it for the other beam. 

A prism made of two pieces of calcite cut in a certain way and cemented together has the property of permitting only one beam to pass through the other is reflected to the side and absorbed in the darkened side of the prism. Such a prism (called a Nicol prism) can be used to form a beam of 

The French physicist Jean Baptiste Biot (1774-1862) then found that some liquids are optically active (that is, have the power of rotating the plane of polarization). For example, turpentine was found to be levorotatory, and an aqueous solution of sucrose (cane sugar, C12H22O11) was found to be dextrorotatory. The substances that were found to be optically active in solution were all organic compounds, produced by plants or animals. 

A puzzling observation was then made. It was found that two kinds of tartaric acid were deposited from wine lees. These two kinds of tartaric acid are closely similar in their properties, but they show the astonishing difference that one is dextrorotatory, and the other is completely without rotatory power. How could there be two molecules with the same composition but with such greatly different power of interacting with polarized light  

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