Mapping the periodic landscape of elements.
When you stand too close to a painting by Monet or Van Gogh, the overall image seems to vanish in a thicket of individual brush strokes. Scientists who have habitually pressed their noses against chemistry's best-known icon -- the flat periodic table of elements -- may similarly have missed a third dimension that transforms the table into a more informative landscape, says theoretical chemist Leland C. Allen of Princeton (N.J.) University.
Allen's suggestion of a 3-D periodic table springs from his reinspection of a fundamental, much used, yet fuzzily defined chemical concept known as electronegativity, which Linus Pauling introduced in 1932. Pauling, who now heads his own research institute in Palo Alto, Calif., describes electronegativity as "the strength at which an electron is held by an atom in a bond." Chemists use the electronegativity values of various elements to determine whether atoms or groups of atoms will combine, and if so, what kinds of bonds -- ionic, covalent or metallic -- will form between them.
Allen prefers to think of electronegativities as "configuration energies" that collectively represent a third dimension of the periodic table. In the standard two-dimensional view, the table's row (horizontal) dimension specifies the number and arrangement of bonding electrons that occupy an element's outer, or valence, electronic shells, the region of all chemical bonding. The column (vertical) dimension corresponds to the size of these shells, which depends on the total number of shells an element has.
When configuration energies emerge as a third dimension, the periodic table "comprise[s] the small set of rules and numbers that help rationalize the observed properties of the 10 million known compounds," Allen writes in the Dec. 6 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. The proposed conceptual integration should improve the table's performance as a chemical pattern recognition scheme, he says.
To determine an element's configuration energy, Allen uses readily available, high-precision values of the element's ionization energies. These values correspond to the amounts of energy required to remove bonding electrons from an atom's valence electron shells. Plugging them into an equation yields the element's configuration energy. Allen says scientists can use configuration energy in more complex calculations for predicting how specific chemical reactions might procced.
Since Pauling first presented his electronegativity scale, others have proposed quantitative refinements of individual values or different definitions of electronegativity. Some of these amendments have stuck, but Pauling's values remain among the more widely used. Although several theoretical chemists say Allen's configuration-energy concept offers a thought-provoking perspective on electronegativity, they remain unconvinced that it will prove more useful than standard values and definitions of electronegativity.
"He has an interesting idea for extending the periodic table, but I'm slightly skeptical about it," comments Roald Hoffmann of Cornell University in Ithaca, N.Y. Robert G. Parr of the University of North Carolina at Chapel Hill also reserves judgment on Allen's arguments. Parr, who has developed a rival method for deriving electronegativity values, says the scientific community will eventually decide which definition of electronegativity reveals the most about chemical bonding.
Allen says he expects a rough reception for his idea because the concept of electronegativity is so central to modern chemistry. However, he stresses that his definition, unlike most others, explicitly recognizes the periodic table's intrinsic energy dimension. In practice, he adds, many chemists have culled information from this dimension without acknowledging its existence.
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|Date:||Dec 16, 1989|
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