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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Eric Scerri wrote:

Julio Gutirrez system

I would like to change the subject for a moment to highlight the work of Julio Gutirrez from Peru. He was at the meeting I just attended in Buenos Aires and explained his ideas to me. Can I suggest that members of this forum take a close look at what he has to say. His system is featured on Mark Leach's website at, _____ made a brief appearance on the present forum but did not follow up.

Tue Jul 19 15:46:31 UTC 2011

Eric Scerri wrote:


There are articles by Valentin Ostrovsky in Foundations of Chemistry and other journals where he gives the original sources for the rule that became known as Madelung's rule. This is not quite as mysterious as Stowe. ____ Maximum n + l value is less precise instead of being more precise when describing electronic configurations. Why choose the less precise description? Maybe because it happens to help one's preferred system of representation, which as I tried to say is not even threatened by the 20 or so exceptions.

Tue Jul 19 15:39:45 UTC 2011

Valery Tsimmerman wrote:

Madelung Rule

My approach to this is following. There is obvious natural phenomenon regarding the order of orbital occupation by electrons. No question about this. It was detected empirically from spectroscopic analysis. It is up to us, humans, to formulate the rule. In order to do that, first we need to understand what that phenomenon is and than choose right words that describe that phenomenon the best way. I do not think it was done in case of n+l rule. I agree with Eric that notion of "differentiating electron" is flawed. It is artificial, just as Aufbau process that does not occur in nature. But what is not artificial are energy/angular momentum levels, or n+l levels, which, when completely filled, jump to next value in quantum manner. Each of those levels represent a natural period. As Philip noted, it was first observed by Charles Janet (or, perhaps, Karapetoff). Janet used it to build his LSPT. If this approach is considered, words "Maximum n+l values attained by the electrons" provide better description of the phenomenon than "orbital occupation by differentiating electron". Just a thought.

Tue Jul 19 14:24:21 UTC 2011

Eric Scerri wrote:


To be clear that was a response to Valery.

Tue Jul 19 12:41:15 UTC 2011

Eric Scerri wrote:

Madelung Rule

Thanks for your posting on the Madelung Rule. I have had further thoughts about this following our conversation on Skype.____I think that your proposal is "too easy" by which I mean it is too easy to remove the apparent exceptions to Madelung's rule by taking the maximum value as you suggest. ________ Why paper over these exceptions just because it avoids irregularities in some representation? This seems the wrong way to proceed. We should concentrate on chemical and physical phenomena. not on trying to preserve some particular presentation from exceptions.____Second point, I disagree about the exceptions being at Mn and Zn rather than Cr and Cu.____The exceptions occur when there is a rearrangement such that 4s2 becomes 4s1 for example, as in both Cr and Cu. Again you may be redefining things to suit your particular purposes.______THird point. The term differentiating electron becomes ambiguous when we are dealing with cases like Cr and Cu. Is it the 4s electron on a 3d that is differentiating? It is both.______Fourth point. The Madelung Rule does not claim this level of precision. In the case of Cr for example the fact remains that the 4s orbital is preferentially occupied and the same is true for Mn, Cu and Zn incidentally. ____Eric Scerri

Tue Jul 19 12:30:09 UTC 2011

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