Periodic Debate

  • DOI: 10.1002/chemv.201000093
  • Author: David Bradley
  • Published Date: 09 June 2011
  • Copyright: Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
thumbnail image: Periodic Debate
Complete, But Not Finished

Non-chemists, and perhaps a few chemists, might have assumed that once all the holes in Mendeleev's Periodic Table were filled with modern discoveries and the lanthanides and actinides added, that the Table was forever immutable, a stone tablet to adorn high school chemistry lab walls, textbooks and websites unchanged forever more ...

Well, they'd be very wrong, aside from the recent didacts on atomic masses and isotope ratios wrought on the elements in December 2010 by IUPAC and the official recruitment of elements 114 and 116, there are several issues that have got many chemists in a boiling reflux.

For instance, there are 3D PTs, spirals, circular tables, stepped and even fractal tables (Fig. 1). Eric Scerri, University of California Los Angeles, USA, is developing an alternative approach to that is intuitive and might take us closer to an ultimate version. Scerri's argument for change is based on the fact that the Periodic Table arose from the discovery of triads of atomic weights, but he thinks chemists would be better served if they were to recognize the fundamental importance of triads of atomic number instead. His new Periodic Table could be fundamentally closer to the ideal.

This is perhaps especially pertinent given that atomic mass varies according to isotope ratio (neutron count, in other words), whereas atomic number (proton count) is fixed for each element. In it, listings of electron shells follow an ordered pattern, so that the halogens form the first column on the left, topped by hydrogen, the noble gases are the second column, topped by helium. The alkali metals and the alkaline earth metals follow, then the block of transition metals. The semi-metals and the non-metals then form the final four columns (Fig. 2).

Figure 1.  Some alternative Periodic Table designs.

Positioning Helium

As if this restructuring of the groups were not controversial enough, it is the logical relocation of hydrogen and helium that stirs deep chemical emotions, even though they recreate the atomic number triads of He-Ne-Ar and H-F-Cl invisible in the conventional PT. However, not everyone is convinced by helium's placement. US chemist Henry Bent would prefer to see helium atop beryllium in the otherwise "normal" PT layout. He argues that although helium seems to fit perfectly at the top of the noble gases its presence there breaks several of the rules.

Scerri is quite adamant that there is one true and objective periodic classification but others believe that such an ultimate PT does not exist and that our perspective inevitably distorts reality. Software engineer Melinda Green from Superliminal Software, developed a fractal PT for educational use and believes any arrangement is purely subjective. "Neither the periodicity nor any classification is intrinsic to nature," explains Green.

Atomic number is perhaps the only intrinsic property of the elements, as suggested by Scerri too, but, adds Green, this is only fundamental by our subjective definition of the term "element" rather than it representing something ultimate about the universe as Scerri's reasoning would suggest. "Every description requires a describer," says Green. "Subjectivity is not just an annoyance, it is the source of all meaning."

Art and Function

So, is the menagerie of different PTs, nothing more than an art gallery? Martyn Poliakoff thinks so. Poliakoff is a professor of chemistry at the University of Nottingham, UK, working on supercritical fluids who has gained recent fame for the Periodic Table of Videos project. His is a pragmatic perspective. "I regard the PT as a tool like a hammer and, just like other tools, you have different forms for different purposes (e.g., a claw-hammer and a mallet). There just isn't a "right" and "wrong" form," he told ChemistryViews. He suggests that the different forms can be useful, however. "These weird forms of the PT often serve a purpose by highlighting some aspect of the elements that one might not otherwise have noticed," he adds.

However, Scerri is convinced there is something more fundamental to the ultimate PT. "It concerns me that scientists can express 'relativistic' [aesthetic] views on something as important as the Periodic Table," he says. "It is after all the most profound and deep classification that has ever been discovered." But Poliakoff has the last word: "In the end, I think that one should remember that Mendeleev devised the PT for a textbook to help rationalize the mass of facts in inorganic chemistry," he adds, "For me, the PT remains a tool to help reduce the complexity, not a metaphysical truth that has a correct form yet to be discovered."

Figure 2. Scerri stuff indeed — a new, rearranged Periodic Table.

► Read on:

At Last, A Definitive Periodic Table?, David Bradley

20 July 2011 — ChemViews article and ensuing discussion has spawned a development in this field courtesy of UCLA chemistry professor E. Scerri

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Jess Tauber wrote:


That was meant to be a pun... Anyway, I mean the usual definition. I just dislike the fact that it mucks up the order of shell filling as demanded by quantum numbers in terms of full orbitals from a purely linear perspective, although of course it leaves the relative orders unfazed. Numericity-wise the numbers are still Pascal-related, but the sequence is messed up. If one uses extra dimensions, though, the issue disappears. On the plus side my first name does begin with a J (a nod sort of to HeBe vs. Henry Bent). Hope your South American trip is joyful. The language I work on, Yahgan, is/was spoken in Tierra del Fuego. IF they had a word for atomic nucleus it would have to be something like yekachinush matekienneka:ki (tiny-nit/pearl (that) can't be seen-the). Jess Tauber

Sun Jul 17 23:01:40 UTC 2011

Eric Scerri wrote:

Spin - Orbit Coupling

Thanks for your input Jess.______You speak about S.O. coupling as though it were some kind of unfortunate disease in the nucleus. Can you clarify what YOU mean by this term?________Eric

Sun Jul 17 15:39:25 UTC 2011

Philip Stewart wrote:

Hocus pocus

The Golden Ratio (not the Golden Rule - that is ethics!) is not hocus pocus; it turns up in all sorts of natural structures. I personally think the "island of stability" is hocus pocus for ultra-massinve nuclei. Magic numbers work up to 126 for Pb, but for non-spherical ultra-massives there is no reason to expect any further magic. The way they spit out alpha particles they are clearly "desperate" to increase their N/Z ratio. A regression of N against Z for the stable nuclei passes far above all the super-massives that have been synthesized. Continuing the periodic system beyond element 120 seems pure fantasy.

Sun Jul 17 06:18:44 UTC 2011

Jess Tauber wrote:


Insomnia.... Spin-orbit coupling becomes a big issue re high Z elements, as for example see the recent predictive work by Pykko. Because these elements may only exist for fractions of a second, we really can't see their full behavioral spectrum. There may yet be surprises. Certainly SO coupling means that any fitting of them to the standard block by block tabular style PT models is a gross approximation only, and only due to quantum number constructional features only. Here is where multidimensional considerations might have some advantages, as they well might also when looking at nuclear shell filling, where spin-orbit rears its ugly head much earlier as an issue. Yet splitting via SO DOES appear to use Pascal Triangle value (doubled) differences, as laid out at the Yahoo T3 group. That is, Pascal shows up both w/wo SO. Not hocus-pocus, just simple combinational math, on the level of cellular automata. PT numericity IS Pascal. And Pascal has direct links to the 'Golden' numbers. But actual energy level values depend more on (perhaps) continuous, relativistic factors, etc. Here Pascal ain't so obvious, if applicable at all. Maybe something else we haven't seen yet? Other 'metal' means? Which would be 'deeper' than Pascal, and more general? There is still wiggle room in the sciences, allowing for proliferation of ever more unlikely explanations (as perusal of arXiv papers will attest). Whatever happened to Occam's Razor? Jess Tauber

Sat Jul 16 19:29:48 UTC 2011

Eric Scerri wrote:

high Z possibilities

Dear Philip and others on high Z, Am still in Buenos Aires but Internet access is rather excellent here. Free in all hotels, restauarants and cafes and usually no need for a password! We should always be prepared for surprises. I think that we should look to theories of nuclear stability for predictions rather than such New Age hocus-pocus stuff as the golden rule. Moreover, the fact that these very high Z nuclei only survive for a small fraction of a second is not the point. Serious chemistry has been conducted on some of these elements and so it is still a worthwhile pursuit. In my forthcoming book (Very Short Introduction to the Periodic Table) I discuss experiments on Z = 107 or Bh and how they helped to establish that the periodic table is still valid up to these high numbers. Eric Scerri______________

Sat Jul 16 15:58:34 UTC 2011

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