Memorial University St. John's. AiC5X7, Newfoundland,Canada
To the Editor:
I wish to raise a point from Ogdvie's excellent attack on conventional wisdom concerning the role of quantum mechanics in chemistry (1).The author correctly points out that if we ignore interelectronic repulsion and assume hydrogenic orbitals in a many-electron atom, then the aufbau principle incorrectly predicts that nickel, to take his example, would be expected to have a full outer shell of electrons. As Ogilvie points out this is a highly unphysical model and one can do better through the Hartree-Fock approximation. I would like to argue that even in the case of this more sophisticated model, nobody has yet explained in detail why the 4s orbital is preferentially occupied although Hartree-Fock calculations show that the 3d orbital has lower energy To the best of my knowledge the only author to take t h s issue seriously is Pilar, in an article in this Journal (21and in his book t3). Pilar has armed .. that the explanation routinely given by textbooks is incorrect. Accordine to the Hartree-Fock aooroximation. the 4s orbital of itsekdoes not have a lowerenergy than 3d. He emphasizes this point by quoting the orbital energies for 3d and 4s for all the elements from scandium to zinc (3). The reason for the oreferential occu~ationof the 4s orbital in the f r s t transkion is due to the peculiar form of the HartreeFock approximation. The energy of each orbital is obtained by considering the interaction of each electron with all other electrons in the atom. As a result there is a double counting that includes the electron repulsion twice. This mav be corrected bv subtracting elec- the aoorooriate -tron rephsion term f& each calculated orbital energy. This last stage is not carried out by Pilar, and he merely argues in terms of plausibility. The 4s orbital penetrates more clmelv to the nucleus, we are told, and so contains electrons thBt are closer to each other and therefore should experience greater interelectron repulsion than those in a I t -~ is therefbre 3d .-orbital. -~ ~ ~ -olauslble ~ - that ~ the subtraction of the greater repulsion term in the case of the 4s orbital will produce a n overall more stable configuration than the analogous subtraction from the 3d orbital. I t is rather unfortunate that the calculation is not actuallv carried out since we are leR wondering whether the the& does actuallv ~ r e d i c tthe confieuration that matches the soectroscopic observation of a 'D ground state term, for exAample, in the case of scandium. All too often, discussions of the reducibility or otherwise of chemistrv to ouantum mechanics revert to whether the reduction is possible in principle. The more relevant question is surelv whether reduction has been achieved in Dractice. I n strictly quantum mechanical terms the electronic configuration of a many-electron atom is meaningless, since individual electrons are not in stationary states (4). However the question may still be asked within the HartreeFock approximation in which orbitals can be defined. More ~
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Journal of Chemical Education
sophisticated quantum mechanical calculations will fail to give the answer to my question since they go beyond the notion of a single c o d i r a t i o n for an atom by admitting configuration mixing. If we claim that quantum mechanics can predict the number of outermost electrons in a n atom then we are bound to choose an approximation in which electrons in shells or orbitals have a well-defined meaning. The most widelv used such method is indeed the Hartree-Fock modei The next questlon along these lines m~ghtbe to exolam the order of orb~talfilline m chromlum and comer. in h e w of the calculated order>f orbital energies ;these atoms. The usual aooroach consists in introducine further ad hoe elements, namely Hund's rules, to obtain &e configurations that agree with spectroscopic data. Whether or not the aufbau principle is a n illusion, a s Ogilvie calls it, should be thoroughly investigated. The answers to these kinds of questions are important to chemical education due to the increasing tendency for teaching the subject from an allegedly quantum-mechanical point of view. Do we really mean quantum mechanics (or some reasonable approximations such as Hartree-Fock), or do we mean quantum mechanical approximations accompanied by all sorts of ad hoc devices and readjustments to fit the experimental data. Chemical education should a t least be clear about the claims made for the role of quantum mechanics in chemistry. Literature Cited 1. O%lvie,J. F J Chom Educ 1890,67,280. 2. Pilar, F L. J. Cham. Educ 1W8.55.2. 3. Pilar, F L. Elamnfory Quantum Chamiatry; MICraw-HillNew York,1990: p 498-
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4. Seemi, E. R. J. Cham Educ. lsBs,66,481.
Note added in proofs: P. G. Nelson has now camed out a detailed calculation of the order of orbital filling in scandium, Educ. in Chern. 1992,29, 88.
E. R. Scerri H.P.S., King's College London WC2R 2LS, England
Replacing Carbon Tet: Other Alternatives
To the Editor: Anyone considering following the recommendation of Petrovic and Bodner [1991, 68, 5091 to replace CCb with o-xvlene in the traditional exoeriment to determine the kqhibrium constant for the formation of 13- may be interested to know that hexanes (a mixture of isomers) also is a satisfadory substitute. Over a decade ago I advocated in this Journal 11978.55.7951 the move to a solvent less toxic than band deschbed o& use of hexanes. Available data indicate that. overall. hexanes and D-xvlenehave comvarable toxicities'with hekanes being more volatile while >-xylene has lower threshold limits. We have used the hexanes version for over 15 years with good results and without incident. We also recover the hexanes by washing i t with aqueous thiosulfate solution followed by drying and distillation. Martin N. Ackermann Oberlin College Oberlin, OH 44074
