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A periodic table for molecules.

A periodic table for molecules

Back in the 19th century, D.I. Medeleevsystematized chemical knowledge of his day by drawing up the periodic table of elements. By arranging the elements according to two properties, atomic number and chemical valence, he could show why elements come in families that make compounds preferentially with certain other families. Later work showed how Mendeleev's periodic arrangement arises from the electronic properties of atoms and so is the key to an understanding of basic chemistry.

Now, Mendeleev's work is being extendedto molecules, Ray Hefferlin of Southern College in Collegedale, Tenn., told last week's San Francisco meeting of the American Physical Society and the American Association of Physics Teachers.

Work at Southern College since 1977has shown that the periodic law applies at least to molecules made of two atoms. Their chemical properties can be predicted, and they form periodic families just as the elements do.

On Mendeleev's chart each elementhas a location in a specific row and column, and that location determines what it does chemically. Mendeleev's chart, having only two dimensions, could be drawn on a sheet of paper. But for two-atom molecules, four dimensions are required -- the row and column addresses of each atom in the molecule. A four-dimensional chart cannot be drawn on paper with any ease, although Hefferlin says his students have produced some interesting computer projections to visualize it.

Hefferlin says the project is ideal workto do at an undergraduate college without sophisticated experimental equipment and with undergraduate students who are not experienced in advanced research techniques. It requires collecting and correlating a lot of data on chemical properties of compounds and involves a lot of hand calculation and computer feeding.

One important thing about a computermemory is that, unlike the human eye, it does not care how many dimensions there are. If the same periodic principle applies to three-and four-atom molecules as to those with two, it will presumably involve even more dimensions. The researchers at Southern College are now working on them.
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Author:Thomsen, Dietrick E.
Publication:Science News
Date:Feb 7, 1987
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