J. Phys. Chem. 1993, 97, 5786
5786
COMMENTS the structureof matterangNot everyonewill share this puzzlement, however, since as I have argued elsewhere4vs the success of the periodic table does not rest on accepting a reductionist account Eric R. Scerri of chemical phenomena and the structure of matter in terms of King’s College, London, The Strand, London WC2R 2LS, quantum mechanics and electronic configurations. I do not think United Kingdom I am alone in claiming that modern physics has not altered the periodic system in any fundamental way.6 Received: September 9, 1992 Allen’s proposalthat the configurationalenergy (CE) represents the missing third dimension is simply misleading, since he is not I would like to raise some points arising from a recent article seriously proposing a 3-D representation of the periodic system by L. Allen] and some other views which he has publi~hed.~.~ but is merely punning on the word dimension. The author proposes In an earlier article, Allen claims that energy should be regarded that his new ‘dimension” should rank alongside atomic number as the missing dimension in the periodic table and that its use can in the arrangement of the periodic table. One might suppose resolve several longstanding anomalies.2 The main reason he that such a quantity would need to be simple and fundamental gives for selecting energy to play this role of the new ‘dimension” if it were to fulfil such a task. However, the proposed quantity is the following: Firstly, he states that the two currently used possesses neither of these attributes. It consists of a complicated dimensionsare atomic number, 2,and the change in shell number function of weighted averages of experimentally determined down any column. Allen then writes, “It is most likely that this ionization energies, and more importantly, it is not fundamental third new dimension is energy because the Schr6dinger equation since it rests on the independent electron approximation in itself identifies energy as the central parameter for describing the specifyingthe number and type of orbitals occupied by thevarious structureof matter.” Themere fact that theSchr6dinger equation valence electrons.5 is concerned with energy is neither a necessary nor sufficient Even if Allen’s configurational energy or his bond polarity condition for making it a new dimension in the periodic table. index turns out to be useful in correlating information on the Furthermore, the energy in question could be total atomic energy, chemical elements, the fact remains that this work consists of an the energy of a particular orbital, or any conceivable function of essentially semiempirical approach with very little explanatory severalorbital energiesof which Allen’s quantity isjust one possible force. In my view we should either use qualitative arguments or example. try to seek deductive arguments. The virtue in qualitative In defining a quantity called bond polarity index, Allen writes, explanations lies in their essential simplicity while the obvious “BecausetheSchriMingerequation identifiesenergyas thecentral virtue in ab initio work lies in its appeal to a deeper physical variable in describing the structure of matter, and because the theory. I do not believe that the middle ground embraced by principal tenet of chemistry is that molecules are made up of Allen offers either of these virtues. atoms, bond polarity is most basically described as an energy difference between atom A and atom B. This differs from the References and Notes long standing assumption that the charge dipole across AB defined (1) Allen, L. C. J . Phys. Chem. 1990, 94, 5602. (2) Allen, L. C. J . Am. Chem. SOC.1989,ll I , 9003. this quantity.”] I believe this to be another non sequitur. In ( 3 ) Allen, L. C. J . Am. Chem. SOC.1992, 114, 1510. another paper the author finds it puzzling that ‘the periodic table (4) Scerri, E. R. J . Chem. Educ. 1991, 68, 122. provides no information about the energy of atoms, even though (5) Scerri, E. R. Br. J . Philos. Sci. 1991, 42, 309. (6) Nelson, P. G. Educ. Chem. 1988, 25, 185. everyone knows that energy is the central parameter for describing
Configurational Energy and Bond Polarity
0022-365419312097-5786$04.00/0
Q 1993 American Chemical Society
