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www.mayanperiodic.com www.rsc.org/eic each kind of atom. electrons orbiting the nucleus of and the describes shell structure of provided by showing similar has been properties, elements fall into vertical columns, The modern explanation why the Quantum mechanical solution providing a secondary classification. procedure may be thought of obtained by the snipping and placing grouping of elements that are primary classification, the vertical number can be thought of as a strips or sequences. elements underneath the previous placing the following sequences of sequence at certain points and ‘snipping’ the one-dimensional representation obtained by table is a two-dimensional ordered elements. But the periodic one-dimensional sequence of the Today atomic number provides a Order for the elements their atomic weights. inverted been if ordered strictly by position in the table would have chemical behaviour but their into place according to their and fell perfectly principle. For example, the elements number provided a better ordering it was later found that atomic increasing atomic weights (fig 1) but originally provided by their mechanics and relativity. and the development of quantum including that of subatomic structure many revolutionary discoveries for almost 140 years and has survived Theperiodic table hasbeen around Eric Scerri source of much debate As chemists we see the as an icon but its design continues to evolve and is the table Trouble in the periodic Where ordering based on atomic The order of the elements was disputed some elements is recognise today periodic table we decide the shape of the ● ● In short ● ● The placement of Various approaches instance, all these metals react with that’s why they behave similarly. For govern their chemical properties and inner shell structures. electron although they differ in their example, all have one outer-shell The elements ingroup 1, for It is the outer-shell electrons which 1s 1s 1s 2 2 2 2s 2s 2s 2 2 1 2p 2p 6 6 3s 3s 2 1 3p 6 4s January 2012 January | 1 and , present articles or on websites. 1000 of them have published been in forms forms of the table. periodic surprising to findso many different Given these basic facts it is a little An abundance of designs beyond to beyond and . from lithium to and potassium as increases one descends the group water, although the reactivity Curiously the first two elements, Education in C h e mi 1 Over s try | 13 1 ,25 3 zhurnal russkeo fiziko-khimchestvo obshchestvo,1871,

something of an anomaly and there alkaline earth metals fall on the left Fig 1 disconnected of elements that has always been some disagreement as a tall block followed by a Mendeleev’s appears as a kind of footnote to the as to exactly which groups they shallower ten element-wide block short-form table. entire system. These elements should be assigned to. In the containing the transition metals. Elements ordered involve the filling of f orbitals. conventional modern periodic table, This is followed on the right edge by according to known as the medium-long form, another taller block, which is six increasing atomic A place for helium these elements fall into groups 1 and elements wide and contains a weights Another more regular 18 respectively – the alkali metals mixture of metals, semi-metals and representation, the left-step table and the noble gases (fig 2). non-metals. (fig 3), was proposed almost 90 The result is a rather awkward In quantum mechanical terms Fig 2 years ago by the Frenchman, twin-tower shape to add to the these blocks are characterised by the The conventional Charles Janet. already uneven shape the periodic filling of (from left to right) s medium-long form Recently this form has enjoyed a table has. The initial unevenness orbitals, d orbitals and p orbitals. of the periodic revival because it also seems to occurs because the alkali and There is a further oddity in the table display the order of orbital filling more clearly than the conventional 2 form.2 It also displays the elements in one continuous sequence without any gaps or interruptions. There is an annoying drawback in that the element helium is removed from its usual place among the noble gases and moved to join the alkaline earths like , and . This move can be justified on the basis of the outer-electron structure of helium, which possesses two such electrons, as do the members of the alkaline earth group. From a chemical point of view, the placement of helium among these metals amounts to complete heresy. While helium is the most unreactive of all the elements in the periodic

14 | Education in Chemistry | January 2012 www.rsc.org/eic 3

table, the alkaline earths are quite Fig 3 suggested that the left-step table Historically, the discovery of reactive. This question of where to A left-step periodic could be modified to avoid the triads using atomic weights rather place helium is one sign of trouble in table heresy of placing helium among the than atomic numbers was the first the periodic table. alkaline earths. This new version hint of any quantitative patterns relocates hydrogen to the top of the among the atomic weights of the Hydrogen doesn’t fit either halogens, while retaining the overall elements. Now in the proposed Even the first element, hydrogen, form of the left-step table (fig 4). modified left-step table, the has been causing trouble for some There is an additional bonus in movement of hydrogen to the time. It can both lose as well as gain presenting the periodic table in this halogen group produces a new an electron. As a result it can be manner. Janet’s original left-step perfect atomic number triad among placed in group 1, as it usually is, or table to the loss of a perfect the elements hydrogen, with the halogens in group 17 (fig ).4 atomic number triad, in moving and . It has even been suggested that helium out of the noble gas group, hydrogen should be grouped at the whereas this new table retains the Trouble in group 3 head of the group since, like helium triad. Another form of trouble lies in carbon, its atom has a half-filled group 3. Many textbook and wall- outer shell. Working with triads chart periodic tables show group 3 Some authors avoid the hydrogen- A triad is the name given to a group as consisting of the elements helium problem altogether by Fig 4 of three elements falling vertically in , , and removing them from the main body Hydrogen in any group of the periodic table. The . A similar number of tables and by allowing them to float above group 17 makes for atomic number of the middle show a difference in the last two the rest of the table. a perfect atomic element is the average of the elements by featuring and More recently it has been number triad elements above and below it. instead.

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www.rsc.org/eic January 2012 | Education in Chemistry | 15 science photo library science photo library been been persuaded to make the change. four elements, not everyb similarity among the second set of analogies in favour of the greater convincing chemical and physical 16 | the notion of to trying maximise the categorical categorical criterion. Although there are some quite What seems to be lacking is a Education in C h 4 Once again e mi ody ody has s try | January 2012 January 3 redesigns... interesting has had some The periodic table firstperiodic table – creator of the Dimitri Mendeleev that the heavier the atom the more few years ago. at least that is how it untilseemed a table, we findeven more trouble, or at the other end of the periodic Moving to the super-heavy elements Bother with the super-heavies and Ac do not (box 1). elements among the group Sc, Y, La triad. However, the last three then the last three form a perfect choice of four elements, Sc, Y, Lu, Lr a criterion. Looking at the second the table periodic may provide such number of atomic number triads in from the last 3 elements from each set. Two possible Box 1 It has known been for some time (39+89)/2 = Ac La Y sets of elements in group 3 of the periodic table and the triads formed 64 (not 57) 39 89 57 anomalously high volatility. this is not what happened. atomic numbers but as it turned out anomalously due to their even higher were expected to behave even more Not synthesised. surprisingly these generalisations. the most general of all chemical that had so far stood the test of time, spoiling a chemical generalisation though relativistic effects were tobeginning peter out. It appeared as chemical was finally periodicity It began to look as if the concept of Crumbling periodicity were conducted. synthesised and chemical experiments other more stable isotopes were extremely short. But after some years as the lifetimes of their isotopes was and ) could not be examined elements 104 and 105 ( synthesised, the chemical reactions of chemical properties. RfCl behave anomalously. For example, make rutherfordium and dubnium effects were significant enough to 1990s showed that the relativistic out carried Experiments in the members, and . expected to behave like its fellow into group 4 and would therefore be in business. that chemical was back periodicity examined chemically, it too showed was and synthesised () later ’. When element 107 included reports the words ‘Boring examined the titles of the news element 106 (seaborgium) were an atom and, as a its result, modify cause distortions to the orbitals of velocities are high enough, this may the inner electrons If also increases. an atomthe increases, velocity of As part. the charge of the nucleus of relativistic begineffects to play a Soon Soon even higher elements were Initially, when they were first For example, rutherfordium falls What had tobegun seem like a When the chemical properties of (39 +103)/2 4 was found to have an Lu Lr Y = 71

A perfect A perfect www.rsc.org/eic 103 39 71 triad science & society picture library Martyn Poliakoff 83 kJ mole prediction would have been basis of the triad principle, the from those of the other two on the tried to predict the value for bohrium 89 kJ mole equal equal length. of them are members of of periods triad perfect if the second and third the table periodic will provide a three elements that fall vertically in www.rsc.org/eic enthalpies of TcO element 107? What does all this have to do with Predictions for bohrium classification more closely. order to examine the question of its has obtained been on this element in consider some quantitative data that Going back to bohrium, we can Döbereiner and triads presence of relativistic effects. table periodic regardless of the still muchvery governed by the higher atomic number elements is concern. The classification of the table turned out to be a short lived threat or trouble for the periodic the of period repeats. thirty-two of period eighteen repeats and then of period eight repeats then the of period two elements, the next from the exceptional first short very occurs in the table. periodic Apart of the twinning of lengths period that explained became as the consequence exact. Theoccurrence of triads only approximate,been became atomic weight, the triads that had periodic system. periodic published the firstfully mature fifty yearsbefore Mendeleev by the chemist Döbereiner, a good The conceptwas initially discovered much longer than table. periodic triads of elements has aroundbeen BhO Box 2 show how the atomic weight of could be calculatedMendeleev’s from use thoseof atomic of the weight four triadssurrounding in twoelements. directions. He published this diagram on several occasions to If we consider the sublimation As As a result of this pattern, any When atomic number succeeded As we have seen, the concept of 3 Cl, the values are 49, 66 and

-1 -1 , which is only 6.7% away respectively. If one had Atomic weights As (75) 5 3 Cl, ReO 3 can be averaged to yield approximately atomic weight of selenium: Cl and (32 + of the elements order of evolution representing the (1888) Crookes’ spiral http://bit.ly/ptsmall a hair table, etched on to smallest periodic The world’s 75 + 80 + 127.5)/4 =79 Te (127.5) S(32) Se? relativistic effects. firmly ingroup 7 of irrespective any the notion that bohrium belongs from the experimental value. calculations. In fact just for good he essentially used triad-like , and scandium, predictions of the properties of When Mendeleev made his famous Two directional triads This small error helps to support Br (80) January 2012 January | chemistry. course many more in the field of atomic and molecular physics as of motivated numerous discoveries in two-dimensional array has also two-dimensional array. This same are all connected together in a , helium, , and synthesised. Substances as diverse as elements so far discovered or relationship between all the 118 all of science. It provides an underlying of the most remarkable discoveries in table encapsulates and displays is one periodic system which the periodic these debates the fact remains that the the alkaline earths. worry about placing helium among extent but chemists understandably regularity is displayed to the greatest many elements. In the left-step table and contains awkward gaps between classified, but the table lacks regularity and helium are seen to be well helium, is still an open question. numbers, namely hydrogen and time being. The trouble at low atomic table has now subsided at least for the atomic number end of the periodic To conclude, the trouble at the high The debate continues table. of certain elements in the periodic problems concerning the placement now in resolving these persistent may be that they can help us again when it comes to the table. periodic It method (box 2). demonstrate the effectiveness of the known and could be used to predicted although it was actually atomic weight of selenium could be directions. measure, he used triads in two 5.  4.  3.  2.  1.  California, US. philosophy of science at UCLA, Eric Scerri is a lecturer in and chemistry References Oxford, OxfordUK: University Press, 2007 E R Scerri, Oxford, OxfordUK: University Press, 2011 E R Scerri, (DOI: 10.1021/ed085p1491.2) W B Jensen, (DOI: 10.1007/s10698-008-9062-5) P Stewart, University Press, 1974 during one hundred years. Tuscaloosa, US: Alabama E Mazurs, Whatever one might think about In the conventional table, hydrogen Triads have an excellent pedigree For example, he explained how the Education in Graphic representations of the periodic system The periodic table, its story and its significance . A very introductionshort to the periodic table. Found. Chem. 2010, J. Chem. Educ., 2008, 12, 5 C 85, 1491 h e mi s try | 17