Rifts in the periodic table / Eric Sherry

The discovery of element 117 filled the last empty slot in the periodic table as we know it today. But along with its completion, it loses its power

In 2010, researchers in Russia announced that they had succeeded in extracting some atomic nuclei of element 117. This new type of atom does not yet have a name, as the scientific community traditionally waits for independent confirmation of the discovery before declaring a new element. But if there are no unexpected surprises, the 117th element has found its permanent place in the periodic table of the elements.

All 116 elements before the new element, and also element 118 which is after it in the periodic table, have already been discovered. Element 117 thus filled the last empty slot in the bottom row of the table. This achievement marks a unique moment in history. When Dmitri Mendeleev, also a Russian, and others created the periodic table in the 60s, it served as the first large map that organized all the elements then known to science. Mendeleev left some empty spaces in the table and made a bold hypothesis: one day, new elements that will be discovered will fill these holes in the table. The table has been corrected countless times, but in all its versions there remain empty slots - until today. Element 19 completes the table in full for the first time in history.

It is likely that Mendeleev's spirit would have reveled in the triumph of his vision, at least for some period of time, until chemists and nuclear physicists were able to synthesize the next elements in line, a feat that would require adding new rows to the table and probably also leaving new empty spaces.

However, while the last missing pieces of the assembly found their place, something much more fundamental began to shake, which could collapse the ground under the idea on which the periodic table itself is based: the periodicity in the properties of the materials that gave the table its name.

Mendeleev not only predicted the existence of the then unknown elements, he also correctly guessed their chemical properties based on this periodic pattern. But as the atomic numbers, indicating the number of protons in the nucleus, increased, some of the other elements stopped behaving as expected according to the principle of periodicity. That is, their chemical interactions, such as, for example, the type of bonds they form with other atoms, no longer resembled those of other elements in the same column of the table. The reason for this is that some of the electrons orbiting around the heaviest nuclei in the table reach speeds that are as high as a significant proportion of the speed of light. These speeds become, in the words of physicists, "relative" speeds, and this caused the atoms to behave differently than expected based on their place in the periodic table. Moreover, it is extremely difficult to predict the orbital structure of each new atom. Therefore, even if Mendeleev's table was completed in its entirety and succeeded, it may have begun to lose its ability to explain and predict the properties of the elements.

Perfect success

Although more than 1,000 versions of the periodic table have been published so far, differing from each other in the arrangement of the elements and even in the elements included in them, they all had one characteristic in common. When you arrange the elements one after the other, according to their atomic number (the first attempts were based on atomic weights), their chemical properties tend to repeat themselves after a certain sequence of elements. For example, if we start with lithium (Li) and move eight squares forward, we will reach sodium (Na), whose properties are similar: both elements are metals so soft that they can be cut with a knife and both react very strongly with water. If we advance another eight slots in the table we will reach potassium (K), which is also a soft metal that reacts with water, and so on.

In the first tables, including those built by Mendeleev but also in others, the cycles, and therefore also the rows in the table, were always eight elements long. However, it soon became clear that in the following cycles, the fourth and fifth, the properties do not repeat themselves after 8 elements but after 18. Accordingly, the fourth and fifth rows are also longer, and the table expands to contain the additional block of elements (the transition metals, which in the common tables are located in the center) . Also, it turned out that the sixth cycle is even longer, and it contains 32 elements. This is due to the fact that another series of 14 elements was added which were called lanthanides, and recently their name was changed to lanthanoids.

In 1937, nuclear physicists began to synthesize new elements. The first was technetium (Tc), which filled one of the four spaces in the table that was known at the time, and which extended from atomic number 1 (hydrogen, H) to 92 (uranium U). The three missing pieces were found a short time later. Two of these elements were synthesized in the laboratory (statin, At, and promethium, Pm) and the third was discovered in nature (francium, Fr). But while those spaces were filled, new discoveries expanded the periodic table beyond uranium and left new empty spaces in their path.

The American chemist Glen Seaborg realized that actinium (Ac), thorium (Th) and protactinium (Pa), together with uranium and the ten elements after it, are a new series, which, like the lanthanoids, has 14 elements and was therefore named actinoids. (Because these additional elements expand the periodic table even further, the conventional tables show the two series, numbering 14 elements each, in a separate block below the main table.)

In the first half of the 20th century, scientists realized that the periodicity of the elements stems from quantum physics, and more precisely, from the physics that explains how the electrons surround the nucleus. The electron tracks appear in a certain variety of shapes and sizes called orbitals. High atomic number atoms have electrons in the same orbitals as lower atomic number atoms, plus electrons in new types of orbitals. In the first cycle, there are electrons in only one type of orbital, known as the s orbital, which can "inhabit" only one or two electrons (one electron in hydrogen, two in helium, He). Also in the second cycle there are electrons occupying one s orbital, as well as in the third cycle. But each of them has electrons in three more orbitals of a new type, p orbitals. And here, each of these four orbitals can occupy one or two electrons, up to the maximum number of eight electrons in all four orbitals together, which is the reason for the periodicity of eight in the original versions of the table. In the fourth and fifth cycles, in addition to the s and p orbitals, there are electrons that occupy a third type of orbitals, five d orbitals, which add another 10 places to occupy electrons and thus stretch the cycle to 18. And finally, the last two cycles also include electrons in one s orbital, in 3 p orbitals, in 5 d orbitals and in addition also in 7 f orbitals, so these cycles include 32 elements (18+14).

When Yuri Oganesian and his colleagues at the Joint Institute for Nuclear Research near Moscow announced their success in synthesizing the elusive element with atomic number 117, all the elements in the last row of the periodic table found their place. The close connection between the structure of the table and the structure of the atoms means that the completion of the table is not only a matter of aesthetics or of arranging the information on a sheet of paper. Element 118 is the only element in which all of its s, p, d and f orbitals are filled with electrons.

If ever more elements are synthesized they will be placed in an entirely new row of the table. Element 119, the next most likely element to be discovered (see table on the previous page), will begin a new cycle - again with one electron in the simplest type of orbital, the s orbital. Yosod 119 and Yosod 120 after which they will be placed in the first two places in the new eighth cycle. But element 121 will start a whole new block of elements, which will involve, at least in principle, a new type of orbitals, which so far have never been populated: g orbitals. As before, the new type of orbital will add new options for occupying electrons and therefore will extend the cycle and increase the number of columns in the table. This block of elements will expand the table to 50 columns (although chemists have already developed more economical ways to organize such an expanded table).

With the completion of the table and the filling of all the rows in it, it seemed that Mendeleev's dream was fully realized. And indeed it might have been so, had it not been for Albert Einstein and his special theory of relativity.

Breaking Bad?

As we progress through the table, from low to high atomic numbers, the number of protons in the nucleus increases and therefore the nuclear charge increases. And as the nuclear charge increases, so does the speed of the electrons in the inner orbitals, to the point where special relativity begins to play a more central role in explaining chemical behavior. This effect causes the internal orbitals to contract and increase their stability. This contraction triggers a chain reaction that affects other s and p orbitals, which also tighten. Among other things, the "valence" orbitals, which are the outermost orbitals, which control the chemical properties, also shrink.

In general, all these phenomena, known as the "direct relativistic effect", increase as the charge of the atomic nucleus increases. But some competing influences complicate matters. While the direct relativistic effect stabilizes certain orbitals, another relativistic effect, "indirect", decreases the stability of d and f orbitals. It is a type of electrostatic masking that the electrons in the s and p orbitals operate because their negative electric charge partially neutralizes the electric attraction of the positive nucleus, as measured far away from it. The electrons further away from the nucleus therefore "feel" a weaker electric attraction, not a stronger one.

We know some relativistic effects on elements in everyday life as well. For example, such effects explain the color of gold (Au), which distinguishes it from the colorless elements that surround it in block d of the periodic table, such as silver (Ag) located in the table just above it.

When a photon of a suitable wavelength hits an atom of a transition metal, of block d, it undergoes excitation. The atom absorbs the photon and its energy causes the electron to jump from the d orbital to the s orbital located directly above it. In a silver element the energy gap between these orbitals is quite large, so a photon in the ultraviolet region of the spectrum is needed to excite the atom. Photons in the visible part of the spectrum, which have lower energy, simply bounce back. We therefore see the metal as an almost perfect mirror.

In gold, the relative contraction works and lowers the energy of the s orbitals at the same time as the energy of the d orbitals increases, thereby reducing the gap between these two energy levels. Now the excitation requires less energy, and this time it corresponds exactly to a photon in the blue part of the spectrum. Photons of other colors continue to scatter, so our eyes receive white light from which blue light is missing. This is why we see the characteristic golden-yellow color of gold.

Pekka Pico from the University of Helsinki and others experimented to predict some of the relativistic effects acting on gold, including the fact that gold atoms are able to bind to other atoms in new and surprising ways. The compounds they expected to get as a result of these interactions were indeed discovered in the end. This is a parallel achievement, to some extent, to Mendeleev's achievements in predicting the existence of new elements. Among Pico's successful predictions are bonds between gold and the noble gas xenon (Xe), usually an incredibly chemically indifferent element, and triple bonds between gold and carbon. Another success is a spherical molecule that includes one atom of the metal tungsten (W) and 12 atoms of gold, which is similar to "fullerene" molecules that contain only carbon, and are better known as "Bucky spheres". These gold fullerenes form quite spontaneously when tungsten and gold are vaporized in the presence of helium gas.

Relativistic quantum-mechanical calculations also proved necessary in the study of the use of gold clusters as chemical catalysts. Gold clusters can, for example, break down toxic substances that are usually released from car exhausts, although gold as a metallic block is known for its chemical indifference.

Super heavy surprises

Even when elements like gold are affected by relativistic effects, they do not deviate to a large extent from their expected nature according to the periodic table. Until recently, the modern elements almost always corresponded to the properties expected of them according to their place in the table. But more serious (and perhaps more interesting) surprises awaited scientists. Some chemical tests of the very recently discovered elements have begun to show signs that serious cracks have begun to arise from the principle of periodicity.

Nuclear physicists create the "superheavy" elements - elements whose atomic number is higher than 103 - by collisions between heavy nuclei in particle accelerators. First experiments carried out in the 90s with the elements rutherfordium (Rf, 104) and dubnium (Db, 105) hinted already then that these elements do not possess the properties expected of them according to their places in the periodic table. Ken Czerwinski and his colleagues at the University of California, Berkeley, for example, found that rutherfordium reacts in solution in a similar way to plutonium (Pu), an element far from it on the periodic table. Similarly, dubnium also showed signs of chemical behavior similar to the element protactinium (Pa) which is distant from it. According to the principle of periodicity, these two elements were supposed to behave similarly to the two elements found right above them in the periodic table, i.e. like hafnium (Hf) and tantalum (Ta).

In later work, the scientists succeeded in synthesizing new superheavy elements only in minute quantities: the discovery of element 117 was based on the observation of only six atoms. Superheavy elements also tend to be very unstable and decay into lighter elements in fractions of a second. Usually all that is left for the experts is to examine the remnants of this nuclear decay and deduce from them information about the physics and chemistry of these atoms. In this state of affairs, studying the chemical properties of these elements using traditional "wet" chemistry, that is, putting the substance in a test tube and observing its reactions with other substances, is out of the question. Still, scientists have found sophisticated methods to study the chemistry of these elements atom by atom.

Chemical experiments conducted on the next two elements were rather disappointing compared to those conducted on elements 104 and 105. It would seem that Cyborgium (Sg, 106) and Bohrium (Bh, 107) behaved exactly as Mendeleev had predicted. This led scientists to come up with names for their research papers such as "the incredibly mundane Cyborgium" and "the boring Boehrium". It would therefore seem that the principle of periodicity is returning to the arena.

In the case of element 112, chemists and physicists tried to assess whether the element behaves like mercury (Hg), which is placed directly above it in the periodic table, or like the noble gas radon (Rn), as some relativistic calculations predict. In such experiments, research teams synthesize atoms of element 112, along with some heavy isotopes of mercury and radon. (Although mercury and radon occur in nature in considerable quantities, researchers prefer to use artificial isotopes because they can produce them under exactly the same conditions as they produce the heavy elements, rather than relying on data based on the macroscopic properties of the lighter, more common elements.)

After extracting the atoms, the experimenters allow them to settle on a surface that is at a very low temperature and coated partly with gold and partly with ice. If element 112 does behave like a metal (that is, like mercury) it will bind to gold, but if it is more like the noble gas radon, it will prefer to sink to the surface of the ice. So far, different laboratories have received different results, so the issue is still far from settled.

The effects of relativity on element 114 are also still unknown. First results reported by Robert Eichler and his team from the Paul Scherer Institute in Switzerland hint at some real surprises in this case, given the rather obvious discrepancy with theory.

New additions to the periodic table are expected, of course, and the study of the chemistry of these elements will help clarify the issue. A more general question is whether there is an end to the periodic table at all. The overwhelming consensus states that when the number of protons becomes too large, nuclei will not form even for a blink of an eye. But opinions differ on the question of where the new foundations will end. In calculations that assume that the nucleus is point-like, the limit appears to be at element 137. Other experts who took into account the volume of the nucleus estimate that the last element will have an atomic number of 172 or 173.

It is still not clear if the principle that elements in the same column in the periodic table behave similarly to each other is also valid for very heavy atoms. This question does not have much practical significance, at least in the foreseeable future. The periodic table's loss of predictive power in the area of ​​the superheavy elements will not affect its usefulness in the rest of the table. The average chemist will never get to play with any of the elements with the highest atomic numbers. Their nuclei are so unstable that after their formation they fade away in the blink of an eye into lighter elements.

And yet, the effects of special relativity go to the heart of chemistry as a scientific field. If the principle of periodicity loses its power, chemistry will be more dependent on physics. If the principle is maintained, this will help the field maintain a certain degree of independence. And in the meantime, Mendeleev's spirit should perhaps just sit back and marvel at the success of his most beloved brainchild.

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About the author

Eric Scerri is a historian and philosopher of chemistry at the University of California, Los Angeles (UCLA). He received his PhD at King's College, University of London and is an accomplished blues guitarist. His latest book is "The Story of Seven Elements" (Oxford University Press, 2013).

The future of the periodic table

The ever-expanding cabinet of chemical wonders

The periodic table organizes the elements according to repeating patterns in their chemical properties. These properties are determined by the orbits in which the electrons in the atom surround the nucleus, or the "orbitals", and especially the electrons in the outermost orbitals. As the atomic numbers increase, the outer orbitals change cyclically. For example, elements 5 to 10 have electrons in outer p orbitals, and this is repeated in elements 13 to 18. All these elements are therefore included in the same block: "p block" (in blue).

New friend, new block

The periodic table in the form shown here is called the Jena table with the left step, named after Charles Jena. Its bottom line will be filled with discoveries of elements 119 and 120, whose outer electrons will be in s orbitals. Element 121 will be the first in which electrons will be occupied in a new family of orbitals, of the g type, and will therefore be placed in a completely new block (bottom left).

Example structures: In lithium (Li), three electrons (not drawn) occupy two s-type orbitals. In pit (B), four electrons occupy two s orbitals and one electron occupies one outer p orbital.

Every two cycles, i.e. every two rows in the table, a new family of orbitals is populated with electrons. On the right are examples of orbital shapes, one of each type.

in brief

The discovery of element 117 Completed for the first time the periodic table as we know it, at least until new discoveries force chemists to expand it and add a new row.

However, the chemical behavior of some of the recent additions may deviate from the behavior of the elements in the same column and break the periodicity principle that defined the table for 150 years.

The surprising behavior probably stems from effects explained by the theory of special relativity, which cause, among other things, the contraction of the orbits of certain electrons in the atom.

Nuclear physicists continue their search To produce new elements, including electrons in new types of orbitals, and to understand their chemistry based on the study of a handful of short-lived atoms.

And more on the subject

• The Periodic Table, Its Story and Its Significance. Eric Scerri. Oxford University Press, 2007.
• A Suggested Periodic Table up to Z ≤ 172, Based on Dirac-Fock Calculations on Atoms and Ions. Pekka Pyykkö in Physical Chemistry Chemical Physics, Vol. 13, no. 1, pages 161–168; 2011.
• A Very Short Introduction to the Periodic Table. Eric Scerri. Oxford University Press, 2011.
• SCIENTIFIC AMERICAN ONLINE See a slide show of the many shapes the periodic table has taken throughout history, plus more multimedia content, at ScientificAmerican.com/jun2013/periodic-table

The article is published with the approval of Scientific American Israel