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Trouble in the Periodic Table Trouble in the periodic table As chemists we see the periodic table as an icon but its design continues to evolve and is the source of much debate Eric Scerri The periodic table has been around for almost 140 years and has survived many revolutionary discoveries including that of subatomic structure and the development of quantum mechanics and relativity. The order of the elements was originally provided by their increasing atomic weights (fig 1) but it was later found that atomic number provided a better ordering principle. For example, the elements tellurium and iodine fell perfectly into place according to their chemical behaviour but their position in the table would have been inverted if ordered strictly by their atomic weights. Order for the elements Today atomic number provides a one-dimensional sequence of the ordered elements. But the periodic table is a two-dimensional representation obtained by ‘snipping’ the one-dimensional sequence at certain points and placing the following sequences of elements underneath the previous strips or sequences. Where ordering based on atomic WWW.MAYANPERIODIC.COM number can be thought of as a primary classification, the vertical grouping of elements that are The elements in group 1, for water, although the reactivity obtained by the snipping and placing example, all have one outer-shell increases as one descends the group procedure may be thought of electron although they differ in their from lithium to potassium and providing a secondary classification. inner shell structures. beyond to caesium and rubidium. Quantum mechanical solution In short Lithium 1s2 2s1 An abundance of designs The modern explanation why the ● Various approaches Sodium 1s2 2s2 2p6 3s1 Given these basic facts it is a little elements fall into vertical columns, decide the shape of the Potassium 1s2 2s2 2p6 3s2 3p6 4s1 surprising to find so many different showing similar properties, has been periodic table we forms of the periodic table.1 Over provided by quantum mechanics recognise today It is the outer-shell electrons which 1000 of them have been published in and describes the shell structure of ● The placement of govern their chemical properties and articles or on websites. electrons orbiting the nucleus of some elements is that’s why they behave similarly. For Curiously the first two elements, each kind of atom. disputed instance, all these metals react with hydrogen and helium, present www.rsc.org/eic January 2012 | EDUCATION IN CHEMISTRY | 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 block 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 beryllium, magnesium and calcium. 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 leads to the loss of a perfect the elements hydrogen, fluorine with the halogens in group 17 (fig 4). atomic number triad, in moving and chlorine. 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 carbon 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 scandium, yttrium, lanthanum and removing them from the main body Hydrogen in any group of the periodic table. The actinium. 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 lutetium and More recently it has been number triad elements above and below it. lawrencium instead. 4 www.rsc.org/eic January 2012 | EDUCATION IN CHEMISTRY | 15 relativistic effects begin to play a part. As the charge of the nucleus of an atom increases, the velocity of the inner electrons also increases. If velocities are high enough, this may cause distortions to the orbitals of an atom and, as a result, modify its chemical properties. Initially, when they were first synthesised, the chemical reactions of elements 104 and 105 (rutherfordium and dubnium) could not be examined as the lifetimes of their isotopes was extremely short. But after some years other more stable isotopes were synthesised and chemical experiments were conducted. For example, rutherfordium falls into group 4 and would therefore be expected to behave like its fellow members, zirconium and hafnium. Experiments carried out in the 1990s showed that the relativistic effects were significant enough to make rutherfordium and dubnium behave anomalously. For example, RfCl4 was found to have an anomalously high volatility. Crumbling periodicity It began to look as if the concept of chemical periodicity was finally beginning to peter out. It appeared as though relativistic effects were spoiling a chemical generalisation SCIENCE PHOTO LIBRARY PHOTO SCIENCE that had so far stood the test of time, the most general of all chemical Although there are some quite The periodic table number of atomic number triads in generalisations. convincing chemical and physical has had some the periodic table may provide such Soon even higher elements were analogies in favour of the greater interesting a criterion. Looking at the second synthesised. Not surprisingly these similarity among the second set of redesigns... choice of four elements, Sc, Y, Lu, Lr were expected to behave even more four elements, not everybody has then the last three form a perfect anomalously due to their even higher been persuaded to make the change.3 triad. However, the last three atomic numbers but as it turned out What seems to be lacking is a elements among the group Sc, Y, La this is not what happened. categorical criterion.4 Once again and Ac do not (box 1). When the chemical properties of the notion of trying to maximise the element 106 (seaborgium) were Bother with the super-heavies examined the titles of the news Moving to the super-heavy elements reports included the words ‘Boring at the other end of the periodic seaborgium’. When element 107 table, we find even more trouble, or (bohrium) was synthesised and later at least that is how it seemed until a examined chemically, it too showed few years ago. that chemical periodicity was back It has been known for some time in business.
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