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The Periodic Table of the Elements, created primarily by Russian chemist Dmitry Mendeleev (1834-1907), celebrated its 150th anniversary last year. It would be difficult to overestimate its importance as an organizing principle of chemistry – all budding chemists are familiar with it from the earliest stages of their education.
Considering the importance of the table, one could be forgiven for thinking that the order of the elements is no longer subject to debate. However, two scientists from Moscow, Russia recently published a new order proposal.
Let’s first see how the periodic table was developed. By the end of the 18th century, chemists were clear about the difference between an element and a compound: elements were chemically indivisible (examples are hydrogen, oxygen) while compounds consisted of two or more elements in combination, having properties quite distinct from their constituent elements.
At the beginning of the 19th century, there was good circumstantial evidence for the existence of atoms. And in the 1860s, it was possible to list the known elements in order of their relative atomic mass – for example, hydrogen was 1 and oxygen was 16.
Simple lists, of course, are one-dimensional in nature. But chemists were aware that certain elements had quite similar chemical properties: for example lithium, sodium and potassium or chlorine, bromine and iodine.
Something seemed to be repeating itself and by placing chemically similar elements next to each other a two-dimensional table could be built. The periodic table was born.
Importantly, Mendeleev’s periodic table had been derived empirically based on the observed chemical similarities of certain elements. It was not until the beginning of the 20th century, after the establishment of the structure of the atom and following the development of quantum theory, that a theoretical understanding of its structure would emerge.
Elements were now classified by atomic number (the number of positively charged particles called protons in the atomic nucleus), rather than atomic mass, but still also by chemical similarities.
But the latter now arises from the arrangement of electrons repeating themselves in so-called “shells” at regular intervals. In the 1940s, most textbooks had a periodic table similar to what we see today, as shown in the figure below.
Periodic table of the day. (Offnfopt / Wikipedia)
It would be understandable to think that this would be the end of the problem. But no. A simple internet search will reveal all kinds of versions of the periodic table.
There are short versions, long versions, circular versions, spiral versions and even three-dimensional versions. Of course, many of them are simply different ways of conveying the same information, but disagreements persist over where certain items should be placed.
The precise placement of certain elements depends on the particular properties we want to highlight. Thus, a periodic table which gives primacy to the electronic structure of atoms will differ from tables whose main criteria are certain chemical or physical properties.
These versions do not differ much, but there are certain elements – hydrogen for example – that we could place quite differently depending on the particular property that we want to highlight. Some tables place hydrogen in group 1 while in others it sits at the top of group 17; some tables even have it in a single group.
A little more radically, however, we can also consider classifying the elements in a very different way, which does not imply an atomic number or reflect the electronic structure – amounting to a one-dimensional list.
New proposition
The latest attempt to order elements in this way was recently published in the Journal of Physical Chemistry by scientists Zahed Allahyari and Artem Oganov.
(Allahyari et al., Journal of Physical Chemistry, 2020)
Their approach, building on the previous work of others, is to assign each element what is called a Mendeleev number (MN).
There are several ways to derive such numbers, but the latest study uses a combination of two fundamental quantities that can be measured directly: the atomic radius of an element, and a property called electronegativity that describes how strongly an atom attracts electrons. towards himself.
If we order the elements by their MN, the nearest neighbors have, unsurprisingly, quite similar MNs. But it is more useful to go further and build a two-dimensional grid based on the MN of the building blocks in what are called “binary compounds”.
These are compounds composed of two elements, such as sodium chloride, NaCl.
What is the advantage of this approach? Importantly, it can help predict the properties of binary compounds that have not yet been made. This is useful in finding new materials that are likely needed for future and existing technologies. In time, there is no doubt that this will be extended to compounds containing more than two elemental components.
A good example of the importance of researching new materials can be appreciated by examining the periodic table shown in the figure below.
Periodic table showing the relative abundance of the elements. (European Society of Chemistry / Wikipedia / CC BY-SA)
This table not only illustrates the relative abundance of items (the larger the box of each item, the more there are) but also highlights potential supply issues related to technologies that have become ubiquitous and essential in our daily lives.
Take cell phones, for example. All the items used in their manufacture are identified by the phone icon, and you can see that several required items are scarce – their future supply is uncertain.
If we are to develop alternative materials that avoid the use of certain elements, the knowledge gained when ordering elements by their MN can prove useful in this research.
After 150 years, we can see that the periodic tables are not only an essential educational tool, they remain useful to researchers in their quest for new essential materials. But we must not think that the new versions replace the previous representations. Having many different arrays and lists only serves to deepen our understanding of element behavior.
Nick Norman, Professor of Chemistry, University of Bristol.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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