More about the Periodic Table

Oct 10, 2009 - Why “should” thorium, [Rn]6d27s2, be best placed in the f-block? It depends on your criteria. Jensen is correct when he speaks of â...
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Letters More about the Periodic Table I found the letter by William B. Jensen about the periodic table most interesting (1), especially his Figure 1, the “periodic table” from Ephraim’s textbook labeled as “Periodisches System der Elemente”. I have an English translation of the 6th edition dated 1954 (2), in which “The Periodic Classification” table now treats elements 57 to 71 slightly differently (see Figure 1 below). Lanthanum is assigned separately in group IIIa under Al, Sc, Y, while the remaining elements, Ce to Lu, labeled “Rare earths (Lanthanons)”, are grouped together but still placed in the IIIa box next to La. In the 25 years that passed between editions, the La–Lu problem was no closer to solution. Another 55 years have now gone by and the debate rages on: plus ça change, plus c’est la même chose. The 1954 table in Figure 1 is interesting for two additional reasons. First, it shows hydrogen standing alone, remote from both Li and F, well removed from C, and above He and the inert gases. If ever there were an element that could be described as sui generis, hydrogen is it. Second, the table in Figure 1 also emphasizes the problem of where to locate the elements post Z = 89. The chemical properties of thorium and uranium are clearly quite different from what is expected from well-behaved lanthanide-like elements with their dominant, robust +3 oxidation states. The actinide hypothesis seems to take effect only from Z =  95 and onwards. Arguing about the “right place” for thorium and uranium (or La and Lu or Ac and Lr) in a f- or d-series seems purpose-

less. Why “should” thorium, [Rn]6d2 7s2, be best placed in the f-block? It depends on your criteria. Jensen is correct when he speaks of “alternative representations” and of the danger of insisting on one, and only one, officially endorsed form of the periodic table. John W. Moore made the point in 2003 (3) and so has Ronald L. Rich more recently (4); there is no ideal or perfect periodic table. Choose a table whose format best suits your needs. It is valuable that the students should see and understand and use the different arrangements. Literature Cited 1. Jensen, W. B. J. Chem. Educ. 2008, 85, 1491–1493. 2. Ephraim, F. Inorganic Chemistry, 6th ed.; Oliver and Boyd: London, 1954; pp 28, 31; revised by P. C. L. Thorne and E. R. Roberts. 3. Moore, J. W. J. Chem. Educ. 2003, 80, 847. 4. Rich, R. L. J. Chem. Educ. 2005, 82, 1761–1763.

Supporting JCE Online Material Keywords; Full text (HTML and PDF) with links to cited JCE articles Michael Laing 61 Baines Road Durban 4001, South Africa Professor Emeritus, University of KwaZulu-Natal [email protected]

Figure 1. The periodic classification from page 28 of ref 2. Note the positions of La (Z = 57) and elements Z = 58 to 71 in the same box; that hydrogen is divorced from the rest of the elements in the table; and the ambiguous positions of the “Actinons”.

© Division of Chemical Education  •  •  Vol. 86  No. 10  October 2009  •  Journal of Chemical Education