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In their consideration of three ways of treating the f-block elements in the standard periodic table, Clark and White opt to footnote a set 15-elements-wide, including both La and Lu, Ac, and Lr (1). Their only grounds seem to be that this is what IUPAC chose in 2005, but that decision sounds like a typical committee fudge—an attempt to please everyone, which in the end will please no one. It is about as unhelpful as the adoption of “lanthanoid” and “actinoid” to replace the time-honored terms. It takes us back to the concept of “rare earths”, which had no regard for electronic configuration. The rationale for dividing the continuous sequence of elements into blocks is that they correspond with the subshells of electrons. Thus, by definition, the s-, p-, d-, and f-blocks should have respectively 2, 6, 10, and 14 elements. If the definition of membership of the f-block is that the differentiating electron of an element could be and is in an f-orbital, only La, Gd, Ac, and Th are exceptions, since neither Lu nor Lr could be distinguished by a 15th f-electron. A slightly weaker generalization is that the nth element in each series has n f-electrons, in which case Yb and No have to be the 14th and last. To this, Pa, U, Np, and Cm are exceptions, but even a generalization that applies to three quarters of the block is worth having. Lu fits into the d-block, as La does not, by sharing with Hf and so forth the lanthanide contraction and its ensuing chemical likeness with its homologue Y. Of the 16 textbooks published since 1982 in Clark and White’s sample, only two opt for a 15-element-wide f-block, and there is an acceleration in the rate at which authors adopt the 14LaAc form.1 It looks as though, if we leave well alone, the solution proposed by Jensen will prevail (2). Alternatively, I have a radical suggestion: a spiral representation of the periodic system can be given an elliptical form such that it fits comfortably on to a standard quarto or A5 page without any need to take the f-block elements out of the continuous sequence (3). The very fact that there is controversy about where to begin and end the f-block (cf. the argument over whether the Zn group are transition elements) is proof that it is a mistake to break the system into sharply delimited blocks. Note- In the 14LaAc representation 14 represents the number of groups in the f-block and La and Ac are the first elements in each row of the f-block.
Literature Cited- Clark, R. W.; White, G. D. J. Chem. Educ. 2008, 85, 497.
- Jensen, W. B. J. Chem. Educ.1982, 59, 634–636.
- Stewart, P. J. Foundations of Chem. 2007, 9, 235–245.
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