Periodic Debate

  • DOI: 10.1002/chemv.201000093
  • Author: David Bradley
  • Published Date: 09 June 2011
  • Copyright: Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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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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5 Comments

bryan Sanctuary wrote:

Rearrange of periodic table.

I recall when I was in freshman chemistry I tried writing the Periodic Table like the spiral above. I could not get it to work well. There are some things I am not sure of from the discussion but the comments are useful. I have no objections to shifting the halogens to be the first period, but H is always the odd man out.

Mon Nov 28 21:56:02 UTC 2011

Bernard Schaeffer wrote:

To Jess Tauber

I don't see what you mean by tetrahedral: is it the structure of the atom or the structure of the table? I used a tetrahedral structure to calculate the binding energy of 4He and now I try to extend this model by an NaCl type crystal structure for the nucleus. Concerning the magic numbers in a nucleus, I think that it is a second order effect because the binding energy curve has only small bumps. To Jess Tauber II don't know Janet's reasoning but like him, contrarily to Bohr and Coster, I don't consider the electrons to build the periodic system. Janet table coincides mathematically with the spherical harmonics (http://storage.canalblog.com/58/52/292736/67078483.jpg). On the last line of this figure you see the series 2, 8, 18 32, all even numbers because of the Pauli exclusion principle. The number 2 is used twice; for H and He and for Li and Be. This is because the symmetry is spherical for these elements (it is also true for Na, Mg, K, Ca etc). For B, C, N, O, F, Ne, there is one plane of symmetry. Dividing by 2 to eliminate Pauli principle we obtain 3 values that are the three possibilities of orientation in space. And so we can continue for all elements without any exception. The electrons don't exist in this model: it is entirely based on the de Broglie wave solved by Schrödinger for the hydrogen atom where there is only one electron. In fact the table desribes only the structure of the hydrogen atom. Adding electrons changes the structure because there are electric interactions between them and, therefore, changes the distribution of the electrons a little for light elements and heavily for heavy elements. Now it is always possible to take the electrons into account but it is incompatible in this type of table. It is necessary to use a table like that of Bohr and Coster.

Wed Aug 03 14:06:59 UTC 2011

Eric Scerri wrote:

LAST WORD

Thanks Philip. See you on the new site.

Tue Jul 26 05:46:54 UTC 2011

Philip Stewart wrote:

Last Word

Nothing much left to say on the new site, except that H-- is like a normal ion, so hydrides are a bit like halides, while H+ is not like a normal ion at all, but covalent H is rather like covalent C. Nothing to do with triads though. Anyway, it's nice to have the last word.

Mon Jul 25 06:41:10 UTC 2011

Valery Tsimmerman wrote:

Triads

I think that significance of atomic number triads, or Z-triads, as Eric calls them, has yet to be completely realized. I do not agree with Eric's position that maximization of number of triads will somehow address the problems of the periodic system. Maximization of number of Z-triads seems to work well for confirming placement of He next to Ne. However, I think that majority of chemists would argue that it does not work as well with placing H next F. Traditional periodic system is the testimony of that. I believe that instead of maximization we should look deeper for basic reasons of double periodicity and, therefore, triads. As Eric noted, the apparent reason for the triads is the fact the lengths of periods recur. Why, then, he attempts to brake this basic regularity by turning first two periods of Janet's LST into one?___ Perhaps, instead of maximization we should look for optimization of number of triads. Take, for example, long form of traditional periodic system, which has first period that does not recur, as the early a version of LST promoted by Eric. It can be observed that first group members of all but two groups do not belong to triads. Out of 32 groups only 2 groups, 2nd and 18th, are exceptions to this rule. Those two groups are the odd balls. Eric's proposition of maximization of triads would increase the number of such exclusions from the rule by another 2 groups. It sure looks like a move in wrong direction. Optimization calls for decrease in number of triads by placing He next to Be. Optimization calls for identical first two periods, just as in Janet's LSPT. ____ Some time ago Jess made interesting observation that every other alkaline earth element, beginning with second element Be matches every other tetrahedral number and all remaining alkaline earths are exact arithmetic means of those numbers. Even Helium, if counted among the alkaline earths, fits this rule if atomic number zero, that Philip calls for in his Chemical Galaxy, is considered. I believe that Eric is correct when he talks about looking at the elements in terms of Z, but it is simplistic to look at triads without regard to the relationship of atomic numbers to the tetrahedral numbers. ____After Jess' mentioned above discovery, I modified Pascal triangle by suppressing all even numbers and using zeros instead of even numbers in one of the two second diagonals (see images at yahoo T3 group). The result was that numbers in fourth diagonal of modified Pascal triangle corresponding to three dimensional space exactly match halves of Z numbers of EVERY alkaline earth element. This type of numerical regularity is much more interesting than triads. Triads are simply outcome of this regularity on a higher level. Therefore, I choose optimization over maximization of triads, since this has much more interesting math at the foundation and remarkably fits the structure, as well as behavior of the elements. ____ I submit copy of this note on the other forum mentioned by Eric.

Thu Jul 21 16:22:57 UTC 2011

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