Misapplying the Periodic Law

Unfortunately criteria 1 and 2 do not always lead to an unambiguous assignment and in those few cases where they fail one must resort instead to criterion 3 to help resolve the impasse. Thus, based on criteria 1 and 2 alone, there are two possible conflicting choices for the two elements below Sc and Y in the first group of the d-block, either La and Ac or Lu and Lr. Both pairs have three valence electrons and an outer (n – 1)d1ns2 valence configuration [the (n – 2)f14 electrons in the latter pair being core electrons, rather than valence electrons]. The gist of my original article was that application of criterion 3 strongly suggested that Lu and Lr were the better of the two alternative choices. In his commentary, Lavelle argues that the above reasoning is outdated based on recent calculations that suggest that Lr does not have an outer (n – 1)d1ns2 valence configuration but rather a ns2np1 valence configuration, thus placing it in the p-block as a heavier analog of Tl. I must confess that this was news to me as I am sure it was to most of the readers of this Journal. As a chemist I do not feel qualified to pass judgement on how seriously one should take such results, but I do know if the results are correct, then they seriously undermine our current understanding of the periodic table. Furthermore, I also know that one cannot consistently argue that these results negate the placement of Lr in the d-block and then turn around, as Lavelle does in ref 1, Note 1, and argue that they can be blithely ignored when it comes to assigning Lr to the f-block! Similarly, it is inconsistent for Lavelle to dismiss (ref 1, Note 4) the (n – 1)d2 ns2 valence configuration of Th as an inconvenient irregularity that should not affect its assignment to the f-block as an idealized (n – 2)f2 ns2 element and then turn around and insist that it is absolutely verboten to entertain the idea that La and Ac may have similar irregular valence configurations corresponding to an idealized (n – 2)f 1 ns2 valence configuration. After all both elements have low-lying empty f orbitals, which is more than can be said for Lu and Lr. Indeed, more than a quarter of the elements in the dand f-blocks have irregular valence configurations and in several instances these irregularities apply to the majority of the elements within a given group. The simple fact is that the periodic table is based on idealized electronic configurations rather than on actual configurations and in this fashion functions in chemistry much as the ideal gas law or the concepts of ideal crystals and ideal solutions. Ultimately I am puzzled as to why Lavelle is so categorically opposed to the idea that La and Ac belong in the f-block. The association of La with these elements has been the dominant view for most of the 19th and 20th centuries and for the case of Ac since the 1940s. There are very good reasons why the other members of the f-block are called lanthanoids and actinoids. Far more serious, however, is Lavelle’s attempt in his third paragraph and in Note 4 (1) to misrepresent my use of group and period property trends by incorrectly equating them with the arbitrary matching of properties for pairs of elements randomly selected from different rows and columns. Self-consistent group and period trends are dictated by the periodic law and by the underlying atomic structure of the periodic table itself, whereas the arbitrary pairing of elements from different rows and columns in the hope that a fortuitous cancellation of trends will lead to property matching is not and is an abuse of the periodic law and table, which violates both criteria 2 and 3. Obviously such an arbitrary “hunt and peck” procedure allows one to prove (or disprove in Lavelle’s case) almost anything. Regrettably this misapplication of the periodic law and table is quite common in the chemical education literature, where it has given rise to some highly problematic results (3–5). Finally, with regard to Lavelle’s assertion in his accompanying letter that he speaks for the silent majority who have been cowed into submission by the “vocal proponents” of the Lu–Lr alternative, I can only say that discussion of this subject is welcome, but for such discussion to be profitable it must be both logically consistent and relevant (6).