Periodic table trends

The Periodic Table of the Chemical Elements is an ultimate and fundamental of nature or merely a tool for creating useful patterns in chemistry. Discuss.

It could be a critical essay title from an undergraduate course in the philosophy of chemistry but it isn’t, it’s a real life periodic debate that rages on year after year among chemists who hope to find an ultimate form for the Periodic Table and those who suggest that any form will do as long as it is useful iin education and research. I recently published a summary of the state of the art regarding such forms and the stance of those who see them as nothing more than aesthetic variations on the theme and those who regard them as taking us a step closer to such an ultimate form for the Table. You can read the periodic debate over how the Table is complete, but not finished in my June Research Highlight column on Chemistry Views. A more in-depth version of the article in which UCLA’s Eric Scerri offers up a radically different PT and in which Martyn Poliakoff of Periodic Table of Videos fame rebuts the notion that such efforts are taking us closer to the ideal Periodic Table.

The aesthetics and ethics of elemental periodicity

It seems that the Periodic Table is in trouble. Well, not trouble exactly but aside from revisions to the atomic weights announced recently and the official adding to the Table of two “new” elements, 114 and 116, the poetically named and fleeting ununquadium and ununhexium, there is a debate bubbling like a reflux condenser about what precisely the Periodic Table is and what form it should take.

For decades chemistry educators and laboratory technicians have sat back and watched the corners of their good-old Periodic Table wall charts curl, each element in its box, trapped forever, with no alchemical chance to move. But that might soon change. Of course, there have been lots of attempts to rebuild the periodic table over the past 150 years. For instance, there are those, such as museum exhibit designer Roy Alexander, who suggests that in the twenty-first century, the 2D is not to be, and that it is high time that chemists made the switch to a 3D format for the Periodic Table. After all, if it’s good enough for Hollywood and Wimbledon, it should be good enough for chemists.

Spiralling into control

There are those who have attempted to create intriguing spiral Periodic Tables, circular efforts and even fractal charts. One Table dating back to the 1930s considers the sub-atomic particle, the neutron, as itself a element and lists the noble gases twice. In the 1920s Charles Janet built a stepped Periodic Table, which was much wider than the standard PT and would inevitably have been less than convenient for textbook publishers and wall chart printers alike.

Others suggest that subtle rearrangements of various elements would make the Periodic Table more intuitive and circumvent various discrepancies that have arisen as nuclear understanding evolved. Chemical philosopher Eric Scerri of the University of California Los Angeles is among that number. He is devising an alternative approach to elemental organization, which he suggests takes us closer to an ultimate version of the PT. Scerri’s argument for change is based on the fact that 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 takes this phenomenon into account.

Scerri stuff indeed

The revision of the Periodic Table to this Scerri form 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. As if this restructuring of the groups were not controversial enough, it is the logical placements of hydrogen and helium that stirs chemical emotions. In relocating H and He, Scerri recreates the atomic number triads of He-Ne-Ar and H-F-Cl; these are not visible in the conventional PT.

However, not everyone is convinced by helium’s placement. Among them is American chemist Henry Bent, known for “Bent’s Rule” of molecular orbitals used by organic chemists and variously written as: “atomic ‘s’ character tends to concentrate in orbitals that are directed toward electropositive groups and atomic p character tends to concentrate in orbitals that are directed toward electronegative groups.” 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. For instance, a Periodic Group’s first member is never the member of a primary (vertical) triad. This rule holds for 30 of the 32 Groups when He is above Ne. The two exceptions are He-Ne-Ar and Be-Mg-Ca. Move He above Be and the rule now holds for all 32 Groups.

Getting the He-Be-gee-bees

The He-Be debate is something of an aside to the philosophical debate that Scerri has unleashed by being quite so adamant that all the various Periodic Table arrangements are moving towards an ultimate version. He doesn’t wish to imply that his version is the essential, final version of which he speaks, but it is perhaps a step closer than the conventional PT we all know and love. “My belief is that there is one true and objective periodic classification even if we have not yet arrived at it,” he says.

Others, such as Philip Stewart, a longstanding fan of the spiral Periodic Table painted by artist Edgar Longman for the Science Exhibition of the 1951 Festival of Britain, based on chemist John Drury Clark’s 1933 original, is not convinced. Stewart argues that to search for “The Ideal Periodic Representation” is to take leave of the messy world of everyday bodies and drift off into Platonic mysticism. Software developer Melinda Green who developed a fractal Periodic Table for educational use agrees and says that an ultimate PT does not exist. Our perspective inevitably distorts reality, she says, any arrangement is purely subjective. “Neither the periodicity nor any classification is intrinsic to nature,” explains Green. “Periods of what? Where do these classes come from? They come from us to suit our particular purposes,” she says.

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. “I don’t believe that there are any ways to describe anything about the universe without a relative position from which to describe,” adds Green. “Every description requires a describer. Subjectivity is not just an annoyance, it is the source of all meaning.”

Chemistry’s rich pageant

Stewart suggests that we should think of the rich variety of images that have been proposed in the last 150 years as “something more like an art exhibition than a competition to achieve perfection. So, is the elemental menagerie, nothing more than an art gallery? Martyn Poliakoff thinks so. Poliakoff is a professor of chemistry at the University of Nottingham, England, who works on supercritical fluids but has gained fame recently for his involvement in a science engagement project known as the Periodic Table of Videos that has gone “viral” on the internet. Poliakoff takes an entirely pragmatic approach to the PT. “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. The different forms highlight different aspects,” he says. He suggests that the different forms can be useful, however. “I think that 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 says.

However, Scerri is convinced that 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 basic, profound and deep classification that has ever been discovered.” The way we perceive the elements and their relationships with each other is fundamental to understanding matter, Scerri believes. The elements are natural entities, they are not building blocks we have constructed for our convenience. The patterns they obey follow objective rules, laws if you will, that are not decided by us and so do not succumb to the whim of the designers of novel Periodic Tables, stepped, 2D, 3D, spiral, fractal or otherwise.

Practical conclusion

Ever the pragmatist, Poliakoff points out a fact of periodic life that may be inescapable in efforts to raise the Periodic Table to some higher position in science. “In the end, I think that one should remember that Mendeleev devised the Periodict Table for a textbook to help rationalize the mass of facts in inorganic chemistry and to make them easier to teach,” he says. “For me, the PT remains just that, a tool to help reduce the complexity, not a metaphysical truth that has a correct form, as yet to be discovered.”

  • Scerri stuff indeed (
  • Two new elements officially added to periodic table (

Catalytic troublemaker

Porous solid catalysts are a mainstay of the modern chemical industry, allowing reactions that would otherwise take an age to progress to be run much, much faster. One group of such catalysts are the zeolites and particularly important among them is one known as ZSM-5, an aluminosilicate material with an MFI structure. However, despite its attractions, ZSM-5 can behave badly because its chemical building blocks do not join together perfectly. This leads to chemical starting materials on which the catalyst is to act often becoming stuck before they can get into the reactive pores and be converted into product. Now, Dutch scientist Marianne Kox has discovered the nature of the miniscule deviations that can make ZSM-5 such a troublemaker.

Catalytic ZSM-5 isn't always on its best behaviour (Credit: Nature Materials/Weckhuysen et al)

Catalysts are essential to the production of a vast array of pharmaceutical drugs, agrochemicals, fuels and countless other chemical products that are made from simple starting materials. Kox and colleague Lukasz Karwacki, together with researchers at the Max Planck Institute for Coal Research in Mülheim an der Ruhr, Germany, ExxonMobil Chemical Europe Inc, Machelen, Belgium, the Centre for Nanoporous Materials, at the University of Manchester, UK, UOP LLC, a Honeywell Company, in Des Plaines, Illinois, USA, and Nicholas Copernicus University, Torun, Poland, have used a raft of spectroscopic techniques, on the micro scale to analyse the structure of zeolite ZSM-5 and have obtained spatial and time-resolved data on the three-dimensional interior of these porous materials. The data reveal the deviations from one porous unit to the next that can lead to reduced efficiency, catalytic poisoning, and unwanted chemical by-products.

Catalytic ZSM-5 (Credit: Nature Materials/Weckhuysen et al)

Kox is working as part of the Vici project run by Bert Weckhuysen, Professor of Inorganic Chemistry and Catalysis at Utrecht University in The Netherlands. Details of the research were published in Nature Materials. The team developed a new approach that correlates confocal fluorescence microscopy with focused ion beam–electron back-scatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped zeolite crystals of differing shapes, sizes, structures, and chemical compositions.

The powerful combination of techniques demonstrates “a unified view on the morphology-dependent MFI-type [zeolite] intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion,” the team say. “It has been found that internal-surface barriers originate not only from a 90° mismatch in structure and pore alignment but also from small angle differences of 0.5 to 2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nanometres.”


Nature Mater, 2009, 8, 959-965

Bert Weckhuysen

Inorganic oil

The stock explanation for the origins of crude oil and natural gas is that these hydrocarbons are the end product of millions of years of geochemical processing of long-dead sea creatures. But, these materials might also be found much deeper in the Earth’s mantle and may have a non-organic origin hinting at a controversial mechanism for a partial replenishment of reserves.

The oil we convert into petrol and feedstocks for the chemical industry, and the natural gas (methane, CH4) that heats our homes started out as living organisms that died, were compressed, and heated under heavy layers of sediment in the Earth’s crust over the course of millions of years. But, the idea that some of these hydrocarbons may have been formed from material that was never alive has been a point of contention for many years.

Anton Kolesnikov and Alexander Goncharov of the Geophysical Laboratory, Carnegie Institution of Washington, in Washington DC and Vladimir Kutcherov of the Lomonosov Moscow State Academy of Fine Chemical Technology, Russia and the Royal Institute of Technology, in Stockholm, Sweden, have studied the possibility of an abiotic origin for hydrocarbons.

“The notion that hydrocarbons generated in the mantle migrate into the Earth’s crust and contribute to oil-and-gas reservoirs was promoted in Russia and Ukraine many years ago,” explains Kutcherov. He and his colleagues have now looked at the laboratory synthesis and stability of hydrocarbons that can form under a range of conditions found in the Earth’s mantle.

They have now demonstrated for the first time that ethane (C2H6) and heavier saturated hydrocarbons, propane, butane, can be formed under the pressure and temperature conditions of the upper mantle, just below the crust. “Our results support the suggestion that hydrocarbons heavier than methane can be produced by abiogenic processes in the upper mantle,” the researchers say.

Artistic view of hydrocarbon formation in the Earth's interior. (Image courtesy A. Kolesnikov and V. Kutcherov)
Artistic view of hydrocarbon formation in the Earth's interior. (Image courtesy A. Kolesnikov and V. Kutcherov)

The team used a so-called diamond anvil cell and a laser heat source to apply a pressure exceeding 20,000 times atmospheric pressure to methane at temperatures from 700 to 1200 Celsius. Under the same conditions ethane was converted to methane. The transformations suggest even heavier hydrocarbons might exist deeper inside the earth. The fact that the processes seem to be reversible implies that saturated hydrocarbons can form from carbon and hydrogen present in the mantle without requiring once-living matter.

If such hydrocarbons formed in the mantle replenish the oil and gas supplies we extract from the crust, then the compounds, which are essentially “reduced” carbon, must survive the migration from the mantle into the crust. How that might happen is not yet clear as the heat and pressure and geochemistry would normally lead to the reversal of reduction, oxidation, to carbon dioxide. As such, Kutcherov believes it is time for a new experimental and theoretical program to study the fate of carbon in the deep Earth and to determine whether migration is possible.

Further reading

Nature Geoscience, 2009, 2, 566-570