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:


I have a question on nuclear structure. In terms of what counts as 'spherical', symmetry-wise, do any polyhedral conformation fit the bill? That is, is a tetrahedron 'spherical' from this perspective, or a cube, dodecahedron, octahedron, icosahedron, or any other regular similar form? Oblate/prolate spheroids/ellipsoids (which count for many if not most of they deformed, superdeformed, hyperdeformed nuclei) have only one linear axis different from the other two. I'm still not clear on all this, and yet it may loom large in terms of the actual packing of nucleons within nuclei, which may NOT be close-packed in all cases, given interpenetration and the odd nature of the strong force. Jess Tauber

Wed Jul 20 23:53:04 UTC 2011

Philip Stewart wrote:

Maximum n+l

Valery's maximum n+l is perhaps the least arbitrary way to answer the old question of whether the lanthanides begin with La or Ce (and the actinides with Ac, Th or Pa). An f electron is not necessary for membership of the 'f block'.

Wed Jul 20 20:09:12 UTC 2011

Valery Tsimmerman wrote:

What is Period?

Philip, you brought you brought up interesting question. The question is: What Constitutes The Period? We have various determinations for groups of elements. The old one is based on similarities of the properties, the newer is based on similarity of the electron structure. We can easily say that two elements belong to the same group if they exhibit similar chemical properties, for example.--- But periods, please correct me if I am wrong, have never been defined clearly. That is the reason why so many different PT arrangements exist and keep popping up. Is there such a rule as "Two elements belong to the same period if ....."? __________ If such rule does not exist, I would like to propose one. Here it is:_____Two elements belong to the same period if their electrons in ground state achieve the same maximum value of n+l. ___Are there any other propositions?

Wed Jul 20 14:21:47 UTC 2011

Philip Stewart wrote:

Gutierrez Samanez

The Quipu or Khipu version assumes a new quantum number, but what do physicists think of that? Recognition of four pairs of periods of equal length is already part of Janet's system, but by making each pair into a single line we lose the adjacency of, for example Na and K, Mg and Ca etc (I know some people want to lose the adjacency of He and Be!). Of course, a spiral representation avoids the need to chop the continuous sequence into strips.

Wed Jul 20 11:11:14 UTC 2011

Valery Tsimmerman wrote:


Eric, Regarding precision, in case of Cr for example which of three electrons is "differentiating": 3d4, 3d5 or 4s1? I could not answer this with precision. So, I came up with more general statement.____ Both 3d electrons do not exist in preceding element Vanadium. Should we call both of them "differentiating"? I thought it should be only one "differentiating" electron per element, corresponding to one additional proton._____ I am not questioning n+l rule in order to justify my table. I truly find this issue confusing.

Tue Jul 19 18:46:18 UTC 2011

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