Chemical Education Today
Letters Which Elements Belong in Group 3? In a commentary published in this Journal Laurence Lavelle argues that the recent trend found in many textbooks, to place lutetium and lawrencium in group 3 of the periodic table instead of lanthanum and actinium, is a mistake (1). Lavelle correctly points out that recent theoretical predictions suggest a new configuration of [Rn]5f147s27p1, rather than [Rn]5f146d17s2, for lawrencium and considers that this configuration weakens the claim that the element should be placed in group 3. It is unfortunate that of the four studies cited by Lavelle (2–5) to support the new configuration of lawrencium only one of them actually does so in a convincing manner while, one does not even mention lawrencium (5), and another one actually concludes against the new configuration (3). However, as Desclaux and Fricke (6) pointed out, lawrencium is the first element for which relativistic effects produce a configuration that is unexpected from its position in the periodic table. One would not dream of displacing silver and gold, for example, from their current positions in the periodic table merely because of the anomalous properties of gold that are also attributed to relativistic effects. The fact that lawrencium might have a configuration that is not strictly analogous to that of the element directly above it in the periodic table is of little bearing on how lutetium and lawrencium should be positioned. More generally, electronic configurations represent a firstorder approximation. An atom does not possess an electronic configuration in a literal sense assumed in general chemistry but exists in a superposition of many different configurations. In addition, electrons in any particular atom cannot be distinguished, which means that speaking of an atom as actually having this or that d electron for example is also strictly an approximation. Lavelle argues that if lanthanum and actinium were to be displaced from group 3, these elements would have to mark the start of the f-block, something that he finds unacceptable since neither atom actually possesses an outer f electron in its groundstate configuration. Lavelle also claims that placing lanthanum and actinium at the start of the f-block would produce “an even worse outcome” because “the entire modern basis of the periodic table”, in his view, includes the notion the f-block possessing 14 outer electrons in seven f orbitals, would be violated (1). However, there is no requirement that every block of the periodic table must necessarily consist of atoms that contain exactly the number and type of electrons predicted from the combination of quantum numbers that are obtained from solving the Schrödinger equation for a one-electron system. For example, much theoretical work has been conducted into which element will feature the first appearance of a g electron. Although there is some debate about this question there is unanimous agreement among theoreticians that it will not take place at element 121 as one might expect on simplistic grounds. The first few elements in what will be termed the g-block will probably not contain a g electron (7)! Better still, consider the configuration of thorium, [Rn]6d27s2. This element follows actinium, and nobody disputes placing it in the f-block even though it does not possesses any outer f electrons. The possession of an outer f electron is clearly not a requirement for an element to be placed in the, nominally named, f-block. I turn now to a final argument that I believe suggests, contrary to Lavelle’s article, that group 3 should indeed consist of 1188
the elements scandium, yttrium, lutetium, and lawrencium. It is generally agreed that the conventional or medium-long form table continues to survive only because it is more conveniently reproduced in textbooks and wall-charts than the long-form table. The medium-long form table relegates as many as 28 elements to a kind of disconnected footnote, and thereby allows one to keep the periodic table relatively slim and having 18 columns. The long-form table, which some textbooks feature, consists of a width of 32 columns (8). But on the plus side it includes the lanthanoids and actinoids in their rightful place within the main body of the table. More importantly perhaps, it maintains an uninterrupted and increasing sequence of atomic number. It is natural to ask what the long-form might look like in using Lavelle’s favored grouping as opposed to the one favored by the present author (9). The answer is that Lavelle’s grouping presents us with a choice between two options. Either the dblock must be split so as to insert the f-block elements between groups 3 and 4 or, if the f-block is inserted between groups 2 and 3, the sequence of increasing atomic numbers is violated. While the first choice is possible, but undesirable, the second one is a nonstarter. Splitting the d-block is undesirable because it only serves to accommodate the wish to group together scandium, yttrium, lanthanum, and actinium. To justify this ad hoc move would require an independent argument, but one that is especially not available to authors such as Lavelle who maintain that the d-block perfectly reflects the filling of five d orbitals by ten outer electrons. Why should there be a break only between the first and second of these electron-filling processes? Meanwhile the grouping recommended by the present author places the f-block between groups 2 and 3 and neither splits the d-block nor violates the order of increasing atomic numbers. Literature Cited 1. Lavelle, L. J. Chem. Educ. 2008, 85, 1482–1483. 2. The Chemistry of the Actinide and Transactinide Elements, 3rd ed., Morss, L. R., Edelstein, N. M., Fuger, J., Eds.; Springer: Dordrecht, 2006; Vol. 3, Chapter 13. 3. Haire, R. G. J. Alloys Compd. 2007, 444–445, 63–71. 4. Fritzsche, S.; Dong, C. Z.; Koike, F.; Uvarov, A. Eur. Phys. J. D 2007, 45, 107–113. 5. Balasubramanian, K. J. Chem. Phys. 2002, 116, 3568–3575. 6. Desclaux, J. P.; Fricke, B. J. Phys. 1980, 41, 943. 7. Hoffman, D. C.; Lee, D. M.; Pershina, V. In The Chemistry of the Actinide and Transactinide Elements, Morss, L. R., Edelstein, N. M., Fuger, J., Eds.; Springer: Dordrecht, 2006; Vol. 3, p 1652. 8. Rayner-Canham, G.; Overton, T. Descriptive Inorganic Chemistry; Freeman: New York, 2006; p 20. 9. Scerri, E. R. The Periodic Table, Its Story and Its Significance; Oxford University Press: New York, 2007; p 17.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Oct/abs1188.html Keywords; Full text (HTML and PDF) with links to cited JCE article Eric Scerri Department of Chemistry and Biochemistry University of California at Los Angeles Los Angeles, CA 90095;
[email protected] Journal of Chemical Education • Vol. 86 No. 10 October 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
