J . Phys. Chem. 1993,97, 5181
Configuration Energy and Bond Polarity Leland C. Allen Department of Chemistry, Princeton University, Princeton, New Jersey 08544 Received: November 19, I992 Dr. Eric Scerri of the History and Philosophy of Science Department, King's College, London, raises seriousand important questions concerning the relationship between physics and the periodic table and its configuration energy extension.' He states, "the success of the periodic table does not rest on accepting a reductionist account of chemical phenomena and electronic configurations. I do not think I am alone in claiming that modern physics has not altered the periodic system in any fundamental way." This viewpoint omits the link that H. G. J. Moseley's 1914 discovery of the atomic number z and Bohr's 1922 introduction of quantum numbers 1 and n and electronic configurations established between the periodic table and the quantum model of the atom. Along with Schrddinger's wave equation, this connection provided chemists, and all other scientists, with a fundamental basis for belief in the periodic table and separated it from the vast array of empirical rules-of-thumb that chemists generate to help organize their experimental observations. Configuration energies take Bohr's advance one step further by recognizing that the energy of electronic configurations can be given as the average ionization potential of the orbitals involved*
CE = (aZ, + b I p ) / ( a+ b ) (1) where a and b are occupancies; I, and Zpare the s and p ionization energies for spherical atoms (for d-block elements Zpis replaced by Id of the n - 1 shell and b is its valence region occupancy). There is, of course, no absolute proof that average ionization energy must be a third coordinate of the periodic table, but it hardly seems arbitrary or surprising since configuration energies (CE) immediately answer three longstanding puzzles of the periodic table: (1) the origin of its diagonal line through the metalloid band which separates metals from nonmetals, (2) the change in properties of the elements as a group is descended in the p-block (metallization), (3) quantitative measureof covalent, metallic, and ionic bonding. All three of these depend upon the strong correlation between CE and energy level spacing^.^ It turns out that configuration energy numerical values almost exactly match (actually adjudicate) the electronegativity scales
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most favored by practicing experimentalchemistsduring the past 60 years, those of Pauling and Allred and Rochow. Since CE is a free atom quantity for which high-accuracy values may be readily obtained and since qualitative conceptual views of electronegativity have an association with ionization potentials, CE appears to resolve the longstanding problem of defining this quantity and in addition making it an intimate part of the periodic table.4 It is identification of configuration energy with electronegativity which gives rise to the definition of bond polarity as the differencein ionizationenergy between an average electron inatomAandoneinatomB. When teaching descriptiveinorganic chemistry, the opportunity to bring together the periodic table's traditional groups along with its quantum mechanically derived properties such as closed shells, I-blocks, bond polarities, and configurationspecification provides a powerful explanatory force for trying to organize the overwhelming amount of available chemical data, and it is difficult to imagine doing without any of these components. In the latter part of his Comment and elsewhere,'VsScerri states that rationalizations of chemical properties should be qualitative, based on a purely empirical periodic table, or deductive, based on high-accuracy Schrddinger equation solutions that go well beyond the Hartree-Fock approximation. While it is true that, for transuranic elements and a few others, there is a loose connection between chemicalbehavior and electronconfiguration: the large majority of chemical phenomena may be semiquantitatively understood in terms of this approximation. In fact, it is indeed the "middle ground", disparaged by Scerri, that we embrace. General chemical trends are frequently hard to discern in the detailed results of high-accuracy electronic structure calculations, and it is only by developing simple, intermediate level, models and concepts, such as configuration energies, that will give us a chance to order and characterize very complex fields like inorganic chemistry. (I wish to acknowledge Eugene T. Knight for helpful discussion of this material.) References and Notes (1) Scerri, E. R. J . Phys. Chem., preceding paper in this issue. (2) Allen, L. C. J . Am. Chem. SOC.1992, 114, 1510. (3) Allen, L. C. J . Am. Chem. SOC.1989, I I I , 9003. (4) Allen, L. C.; Knight, E. T. J . Mol. Struct. (THEOCHEM) 1992, 261, 313. (5) Scerri, E. R. J . Chem. Educ. 1991,68,122. NewSci. 1989 (Feb 1 l), 76. (6) Jsrgensen, C. K. Angew. Chem., Int. Ed. Engl. 1973, 12, 12.
0 1993 American Chemical Society
