Valence
In 1799, Joseph Louis Proust was the first to recognize that chemical compounds may have a definite, constant composition. Proust formally stated a law of constant proportions. This law raised the possibility that elements might have certain characteristic tendencies to combine with each other. One gram of hydrogen, for example, was known always to combine with eight grams of oxygen in the formation of water.
After John Dalton's statement of the atomic theory, this observation could be expressed in terms of atoms. That is, each type of atom appeared to have some specific tendency to combine with other atoms. The first clear statement of this idea did not appear until about 1852, however. One reason for the delay in formulating this concept was that the techniques of chemical analysis were still too crude to yield unequivocal data about the composition of compounds. Also, the relationship among atoms, molecules, and chemical formulas was still not clear.
The research that finally led to a modern theory of valence (or valency) was Edward Frankland's study of organometallic compounds. Organometallic compounds consist of a metal combined with one or more organic groups. Frankland found that each metal he studied combined with only a fixed number of organic groups. For zinc, for example, that number was two. He concluded that "the combining power of the attracting element, if I may be allowed the term, is always satisfied by the same number of these atoms." By combining power, Frankland was referring to the concept that was later called quantivalence, or valency, or valence. It is somewhat surprising that the concept of valence grew out of organic chemistry. Organic compounds are, in general, much more complex than are inorganic compounds. Frankland quickly showed, however, a number of examples from inorganic chemistry that supported his view of "saturation capacity " or "combining power."
The next step in valence theory was Alexander W. Williamson's (1824-1904) idea that certain specific atoms were needed to hold other atoms together in a molecule. He showed that one oxygen atom could hold two hydrogen atoms together in a water molecule, two ethyl groups together in diethyl ether, one potassium and one hydrogen atom together in potassium hydroxide, one potassium and one sulfur together in potassium sulfite, and so on. Although he never clearly talked about the "combining capacity" of an atom such as oxygen, that concept was clear in his work.
The idea implicit in Williamson's research was made explicit by Friedrich Kekulé in 1858. In his analysis of organic compounds, Kekule came to the conclusion that the carbon atom is always tetravalent; that is, it always combines with four other atoms, no more and no less. He and Archibald Couper (1831-1892) were among the first to develop symbols to represent this combining power, or valency, of the carbon atom.
At least one important problem remained with valence theory: the possibility of variable valences. Sulfur, for example, appears to have a valence of two in hydrogen sulfide, four in sulfur dioxide, and six in sulfuric acid. How could one atom exhibit so many different degrees of "saturation capacity," or valence? The problem was especially difficult for chemists who believed that valence was a fundamental property of atoms, like atomic weight or atomic number. A number of ingenious solutions were proposed for this dilemma. Kekule, for example, suggested that atoms might be strung together in molecules in which apparent valences did not represent actual structures. He thought that sulfuric acid, for example, might actually be:
H-O-O-S-O-O-H
, in which sulfur's "real" valence was two, as it was in hydrogen sulfide.
None of these solutions was really acceptable, however, and it was not until the concepts of ionic and covalent chemical bonding were developed in the twentieth century that this problem was satisfactorily resolved.
This is the complete article, containing 622 words
(approx. 2 pages at 300 words per page).
