46 | World | March 2009 www.chemistryworld.org Periodic change The periodic table, cherished by generations of , has steadily evolved over time. is among those now calling for drastic change The periodic table has become recurrences as vertical columns or something of a style icon while In short groups. remaining indispensable to chemists.  In its original form The notion of chemical Over the years the table has had the periodic table was is something of a vague one. To make to change to accommodate new relatively simple. Over this idea more precise, the periodic elements. But some scientists the years, extra elements table pioneers focused on the propose giving the table a makeover have been added and the maximum of each element while others call for drastic changes layout of the transition and looked for similarities among to its core structure. elements altered these quantities (see Mendeleev’s More than 1000 periodic systems  Some call for drastic table, p48). have been published since the table rearrangements, The method works very for Russian Dimitri Mendeleev perhaps placing the elements up to atomic weight developed the mature periodic with the . 55 () after which point system – the most fundamental  A new may be it starts to fall apart. Although natural system of classification needed when chemists there seems to be a repetition in the ever devised. (Not to mention the can make elements in highest valence of and hundreds if not thousands of new the g-block, starting at (3), and systems that have appeared since the element 121 (4), and (5), advent of the internet.) and and manganese (7), Such a proliferation prompts this is not the case with questions as to whether some tables and . Whereas potassium shows are more valid than others and a maximum valence of one, iron even whether there might be one shows a maximum valence of three. optimal periodic system which best It does not fit properly underneath reflects the relationship between the potassium and nor do the next two chemical elements. elements, and . But the The underlying science embodied following element, , does show by the periodic system is deceptively a valence of one and two. simple. If the elements are placed Ignoring the valence of two gives at in a natural sequence according to The periodic table’s least some reason for placing copper increasing there iconic design has taken underneath potassium. Similarly, appears to be an uncanny repetition it outside the realm of fits perfectly underneath in their properties, albeit an science since both elements show a approximate one. Using atomic number the elements can be arranged into a one- dimensional sequence, which is the precursor for any periodic system.

H (1), He (2), Li (3), Be (4), B (5)......

But the periodic table is a two- dimensional display of the elements. It cuts the sequence at particular places in order to reflect the chemical similarities of elements. The resulting strips can be pasted, one underneath the other, to

display the approximate chemical (HTTP://GEOFFJONES.COM) JONESW GEOFF www.chemistryworld.org | March 2009 | 47 Periodic table , 25 , 3 , 1871, 1871, , HURNAL RUSSKEO FIZIKO-KHIMICHESTVO OBSHCHESTVO FIZIKO-KHIMICHESTVO RUSSKEO HURNAL Z maximum valence of two. of the main body of his compact or Mendeleev’s table of . He called these ‘transition The question is what to do with so-called short-form periodic table. 1871 contained a new elements’, because they represented iron, cobalt and nickel? Mendeleev A similar situation occurs with group of elements: transitions in the sequences of solved this problem by creating an , and , group VIII elements which showed similarities anomalous new group, or rather a and later with , and in their maximum valences, before group containing three elements in , which Mendeleev likewise such behaviour was taken up again as each place. He placed the new group, expelled from the main body of the in the case of copper and zinc, which he labelled VIII, to the right table and placed into the anomalous and , and finally and . This simple act of removing certain elements from the main body of the periodic table is key to understanding why it has been represented in so many different forms. Nowadays the most commonly encountered form of the periodic table is the medium– long form (left). Today’s transition elements are not the same as Mendeleev’s – the A question of : the sense in which they are ‘transitional’ medium-long-form table has changed. In order to appreciate (top) or the long-form this change we must consider table (bottom)? the electronic configuration of , none of which was known at the time of Mendeleev and his contemporaries. An of potassium has a configuration of 1s2, 2s2, 2p6, 3s2, 3p6, 4s1 followed by calcium, which is 1s2, 2s2, 2p6, 3s2, 3p6, 4s2. But then something strange happens – the following 10 elements after calcium have a configuration involving the occupation of the 3d orbitals. It is only when this process is completed 48 | Chemistry World | March 2009 www.chemistryworld.org SCIENCE PHOTO LIBRARY PHOTO SCIENCE

several two and three- The periodic table has the conventional table. As long dimensional periodic had some interesting as the medium-long-form prevailed that subsequent systems have been redesigns including as a it appeared as though atoms resume the devised in order to or as these conical and were equally good filling of s and p orbitals. accommodate the transition structures from 1947 candidates for membership to And just as Mendeleev solved the in novel ways group three as were and problem by removing his transition (see above). . But with the growing elements from the main body of But there is still nothing drastic willingness to display the elements in the table, so modern chemists and involved in these arrangements, a long-form table, the choice in favour solved their problem by despite their appearance. Tables of lutetium and lawrencium becomes removing a block of elements: the with more significant variations inescapable. If one places lanthanum d block. But instead of placing these place some elements in and actinium below scandium and new transition elements to the right different groups. One such in a long-form arrangement of the main body of the table, they arrangement is the the sequence of atomic numbers are traditionally placed between two left-step periodic table in is violated in a rather shocking unevenly separated portions of the which is placed at manner. main table, the s block and the p block. the top of the alkaline Finally, it appears that the In the 1940s, American chemist metals rather than at the top recent synthesis of super-heavy Glen Seaborg made a further of the noble . elements up to and including modification to the periodic table, this I suggest a table which element 118, (with the exception time involving the inner transition would place hydrogen of element 117), is reawakening an elements called the . As a among the halogens, resulting interest in how best to represent the result of his work these elements in a new perfect atomic number periodic system. – which had previously been housed triad involving hydrogen, With the possibly imminent advent in the main body of the medium- and chlorine (see below). Triads of the g-block, formally starting at long-form table – were also removed were historically the first hint of element 121, it will become necessary to join the and to create a numerical regularity among the to create a new block in the table. the long-form periodic table elements and many people have Should this be placed as a footnote (see page 48). suggested placing hydrogen among to a long-form table or perhaps All these forms are still fairly the halogens on the basis of chemical incorporated into a very-long-form traditional, although the act of and physical evidence. One suggestion for a table with 50 columns? Needless to removing elements from the main An interesting debate has reformed table is to say, several versions of both kinds can tables does not have to be carried out developed over which elements place hydrogen with the already be found – this is an issue that on a two-dimensional display. Today, should be placed in group three of halogens won’t be resolved in a hurry.

Eric Scerri teaches chemistry and history and philosophy of science at UCLA in , US. He is the author of The periodic table, its story and its significance, Oxford University Press, 2007

References  E R Scerri, Educ. Chem., 2005, 42, 135  E R Scerri, J. Chem. Educ., 2008, 85, 585  W B Jensen, J. Chem. Educ., 1982, 59, 634  www.meta-synthesis.com/webbook/35_pt/pt.html

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