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 (sciencebase.com)
  • Two new elements officially added to periodic table (theglobeandmail.com)

Between a rock and a fluid place

US researchers have found a way to monitor geological faults deep in the Earth that could help them predict an imminent earthquake more precisely than with other methods. This is the first time that scientists have been able to detect temporal changes in fault strength at seismogenic depth from the Earth’s surface.

The late Paul Silver and Taka’aki Taira of the Carnegie Institution’s Department of Terrestrial Magnetism, working with Fenglin Niu of Rice University and Robert Nadeau of the University of California, Berkeley, used highly sensitive seismometers to detect subtle changes in earthquake waves that travel through the San Andreas Fault zone near Parkfield, California, over a two-decade time span.

“Fault strength is a fundamental property of seismic zones,” explains Taira, who has moved to the Berkeley since the research was done. “Earthquakes are caused when a fault fails, either because of the build-up of stress or because of a weakening of the fault. Changes in fault strength are much harder to measure than changes in stress, especially for faults deep in the crust. Our result opens up exciting possibilities for monitoring seismic risk and understanding the causes of earthquakes.”

Seismologists have focused the San Andreas Fault near Parkfield, the “Earthquake Capital of the World,” for years. The site has a sophisticated array of borehole seismometers, the High-Resolution Seismic Network, as well as other geophysical instruments in situ. Researchers consider it a natural laboratory for seismology because of the frequent quakes that occur there.

Earlier studies have suggested that there are fluid-filled fractures within the fault zone and that these shift. When this happens, “repeating” earthquakes apparently become smaller and more frequent, which researchers say is indicative of a weakened fault. “Movement of the fluid in these fractures lubricates the fault zone and thereby weakens the fault,” says Niu.

“The total displacement of the fluids is only about 10 metres at a depth of about three kilometres, so it takes very sensitive seismometers to detect the changes, such as we have at Parkfield.” Niu further explains that it seems to be distant earthquakes that cause the fluids to shift, such as the 2004 Sumatra-Andaman Earthquake, which led to tsunamis throughout the Indian Ocean that year.

It is San Andreas fault (Adapted from Wikipedia image)
It is San Andreas fault (Adapted from Wikipedia image)

The authors speculate that such large events should produce a temporal clustering of global seismicity, a hypothesis that appears to be supported by the unusually high number of large earthquakes occurring in the three years following the 2004 earthquake. The team presents additional evidence that a similar phenomenon occurred following the 1992 Landers earthquake.

LINKS

Nature, 2009, 461, 636-640
Department of Terrestrial Magnetism at the Carnegie Institution of Washington
Northern California Earthquake Data Center (NCEDC)

Cheerio!

Anyone who enjoys a delicious bowl of mixed grain Cheerios in the morning, rather than being force-fed lumpy porridge, will know all about the Cheerios Effect. This phenomenon is usually manifest at a particular point during breakfast when just a few torus-shaped Cheerios are left floating on the milk. Minor agitation of this mixed-phase, composite system will cause individual Cheerios to bump into each other and to form aggregate complexes.

This amazing scientific phenomenon is due to the surface tension of the milk, which is essentially an emulsion of fats and proteins in water. The effect was only recently reported in the scientific literature and ranks alongside, other surface tension phenomena including the ability of water to migrate from one end to the other of a thin tube, capillary action, the ability of some aquatic insects, such as pond skaters, to walk on the surface of water, and the recent discovery of liquid marbles, which can be formed by powder coating solvent droplets.

Dr Steven E J Bell

Dr Steven E J Bell

Now, Steven Bell of Queen’s University Belfast has combined his team’s expertise in superhydrophobic materials, discussed previously in Spotlight, to enable a rather novel demonstration of the Cheerios Effect on a much smaller scale with potential technological applications.

The researchers coated microscopic particles of copper metal with their superhydrophobic material. When you pour these particles on to water they float because of their powerful water repellency, or hydrophobicity, despite the density of the copper. However, they gradually assemble into really strong sheets because of the ‘Cheerios Effect’, which is really powerful in this case, explains Bell. The high density of the particles bend the meniscus, the surface of the water, significantly.

A sugar cube coated with the particles can be held under water without dissolving (Image courtesy of Iain Larmour)

A sugar cube coated with the particles can be held under water without dissolving (Image courtesy of Iain Larmour)

Bell and his colleagues have uploaded two videos to YouTube showing the microscopic Cheerios Effect in action. One video shows the spontaneous coalescence of the particles, the other demonstrates just how strong it is with a view of the flat end of a pencil being pushed through a layer of particles and into the water. One can see that no water is absorbed on to the surface of the pencil, which is not the case when a pencil is dipped into water without the superhydrophobic layer. Other phenomena include the protection of a sugar cube from dissolving in water and the supporting of other objects on the surface of water.

Two coloured water drops sitting on an assembled superhydrophobic sheet, gaps between the particles are obvious but the water does not fall through (Image courtesy of Iain Larmour)

Two coloured water drops sitting on an assembled superhydrophobic sheet, gaps between the particles are obvious but the water does not fall through (Image courtesy of Iain Larmour)

The most striking property of the sheets is the extent to which they can support objects, Bell says, sheets composed of either small (less than 75 micrometres) or large (less than 400 micrometres) particles can support the weight of water droplets indefinitely and even irregular, relatively large fragments of cement.

A cement fragment supported by the larger superhydrophobic copper particles (Image courtesy of Iain Larmour)

A cement fragment supported by the larger superhydrophobic copper particles (Image courtesy of Iain Larmour)

Bell adds that, The superhydrophobic copper particle sheets support objects because they satisfy two linked criteria: First, the particles in the sheet have sufficient buoyancy to support the external load. Secondly, under loading conditions the sheets do not tear or develop holes which are large enough for the supported object to contact the water beneath. Preventing this contact is important, because as soon as it occurs the supported objects fall through the sheet.

The particles may one day have technological applications, but at the moment, Bell and colleagues are optimizing the shape of the particles to make stronger sheets. They are also using their materials as a macroscale model of the microscopic phenomenon of emulsification, in which tiny, colloidal particles suspended in a liquid form a stable mixture that never settles, as occurs in milk.

Working with Iain Larmour and Graham Saunders, Bell has now published details of the work in the chemistry journal Angewandte Chemie with the title: Self Assembled on Water by the Cheerios Effect. I always wanted to get the name of a breakfast cereal into the title of a serious research paper, Bell told Spotlight.

Further reading

Angew Chem Int Edn, 2008, 47, 5043-5045
http://dx.doi.org/10.1002/anie.200705833

Dr Steven E J Bell homepage
http://www.ch.qub.ac.uk/staff/bell/

Grey to green: Spotlight article
http://www.intute.ac.uk/sciences/spotlight/issue42/Grey_green.html

The Cheerios Effect
http://www.youtube.com/watch?v=KS6VZH7PTjs

The pencil being pushed through the sheet
http://www.youtube.com/watch?v=0I4cEtkVuO4

Suggested searches

surface tension
fluids