![]() Their approach, building on the earlier work of others, is to assign to each element what’s called a Mendeleev Number (MN). The latest attempt to order elements in this manner was recently published in the Journal of Physical Chemistry by scientists Zahed Allahyari and Artem Oganov. Rather more radically, however, we can also consider ordering the elements in a very different way, one which does not involve atomic number or reflect electronic structure – reverting to a one-dimensional list. Some tables place hydrogen in group 1 whereas in others it sits at the top of group 17 some tables even have it in a group on its own. These versions don’t differ by much, but there are certain elements – hydrogen for example – which one might place quite differently according to the particular property one wishes to highlight. Thus, a periodic table which gives primacy to the electronic structure of atoms will differ from tables for which the principal criteria are certain chemical or physical properties. The precise placement of certain elements depends on which particular properties we wish to highlight. Many of these, to be sure, are simply different ways of conveying the same information but there continue to be disagreements about where some elements should be placed. There are short versions, long versions, circular versions, spiral versions and even three-dimensional versions. A simple search of the internet will reveal all sorts of versions of the periodic table. It would be understandable to think that this would be the end of the matter. Something seemed to repeat and by placing chemically similar elements next to each other, a two-dimensional table could be constructed. But chemists were aware that certain elements had rather similar chemical properties: for example lithium, sodium and potassium or chlorine, bromine and iodine. Simple lists, of course, are one-dimensional in nature. And by the 1860s, it was possible to list the known elements in order of their relative atomic mass – for example, hydrogen was 1 and oxygen 16. By the early 19th century, there was good circumstantial evidence for the existence of atoms. By the late 18th century, chemists were clear about the difference between an element and a compound: elements were chemically indivisible (examples are hydrogen, oxygen) whereas compounds consisted of two or more elements in combination, having properties quite distinct from their component elements. Let’s first consider how the periodic table was developed. However, two scientists in Moscow, Russia, have recently published a proposal for a new order. Given the table’s importance, one might be forgiven for thinking that the ordering of the elements were no longer subject to debate. It would be hard to overstate its importance as an organising principle in chemistry – all budding chemists become familiar with it from the earliest stages of their education. As news of his remarkable accomplishment began to spread, Mendeleev became something of a hero, and interest in the periodic table soared.The periodic table of the elements, principally created by the Russian chemist, Dmitry Mendeleev (1834-1907), celebrated its 150th anniversary last year. In a prestigious Faraday Lecture to the Royal Institution in London in 1889, he admitted that he had not expected to live long enough ‘to mention their discovery to the Chemical Society of Great Britain as a confirmation of the exactitude and generality of the periodic law’. Mendeleev himself was surprised by how fast his ideas were confirmed. Within twenty years, all three had been found, and their properties confirmed his predictions almost exactly. So convinced was he of the soundness of his periodic law that he left gaps for these elements in his table. Soon, Mendeleev was predicting the properties of three elements – gallium, scandium and germanium – that had not then been discovered. Now each chemical element had its number and fixed position in the table, and from this it became possible to predict its behaviour: how it would react with other elements, what kind of compounds it would form, and what sort of physical properties it would have. Mendeleev’s ideas, which built on the earlier work of French chemist Antoine Lavoisier in the previous century, totally changed the way chemists viewed their discipline. It was perhaps the greatest breakthrough in the history of chemistry. He wrote down the sequence in such a way that they ended up grouped on the page according to known regularities or ‘periodicities’ of behaviour. On 17 February 1869, Russian chemist Dmitri Mendeleev jotted down the symbols for the chemical elements, putting them in order according to their atomic weights and inventing the periodic table.
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