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Nick Norman, Bristol University
The periodic table of the elements, created mainly by the Russian chemist Dmitry Mendeleev (1834-1907), celebrated its 150th anniversary last year. It would be hard to overstate its importance as an organizing principle in chemistry: all budding chemists know it from the earliest stages of their education.
Given the importance of the table, one could be forgiven for thinking that the order of the elements was no longer the subject of debate. However, two scientists in Moscow, Russia recently published a proposal for a new order.
Let’s first consider 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: the elements were chemically indivisible (examples are hydrogen, oxygen) while the compounds consisted of two or more elements in combination, with properties quite distinct from their component elements. In the early 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 relative atomic mass, for example hydrogen was 1 and oxygen was 16.
Simple lists, of course, are one-dimensional in nature. But the chemists knew that some elements had quite similar chemical properties: for example lithium, sodium and potassium or chlorine, bromine and iodine. Something seemed to repeat 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 was derived empirically based on the observed chemical similarities of some elements. It would not be until the early 20th century, after the structure of the atom was established and following the development of quantum theory, that a theoretical understanding of its structure would emerge.
The elements were now sorted by atomic number (the number of positively charged particles called protons in the atomic nucleus), rather than by atomic mass, but still also by chemical similarities. But the latter now stemmed from the arrangement of electrons repeating themselves in the so-called “shells” at regular intervals. In the 1940s, most textbooks featured a periodic table similar to what we see today, as shown in the following figure.
It would be understandable to think that this would be the end of the matter. Not so, however. 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. Many of these, to be sure, are simply different ways of conveying the same information, but there continue to be disagreements over where certain elements should be placed.
The precise placement of certain elements depends on which particular properties we want to highlight. Therefore, a periodic table that gives primacy to the electronic structure of atoms will be different from tables for which the main criteria are certain chemical or physical properties.
These versions don’t differ much, but there are some elements – hydrogen for example – that could be placed quite differently depending on the particular property you want to highlight. Some tables place hydrogen in group 1, while in others it is at the top of group 17; some tables even have it in a group by itself.
Rather more radically, however, we can also consider ordering the elements in a very different way, which does not involve the atomic number or reflect the electronic structure, returning to a one-dimensional list.
New proposal
The latest attempt to sort the elements in this way was recently published in the Journal of Physical Chemistry by scientists Zahed Allahyari and Artem Oganov. Their approach, based on the previous work of others, is to assign to 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 directly measured: the atomic radius of an element and a property called electronegativity which describes the force with which an atom attracts electrons to itself. same.
If you sort items by their MN, the closest neighbors have, unsurprisingly, quite similar MNs. But it is more useful to go a step further and build a two-dimensional NM-based grid of the building blocks in so-called “binary compounds”. These are compounds composed of two elements, such as sodium chloride, NaCl.
What is the benefit of this approach? Importantly, it can help predict the properties of binary compounds that have not yet been realized. This is useful in researching new materials that are likely to be needed for both future and existing technologies. In time, no doubt, this will be extended to compounds with more than two elementary components.
A good example of the importance of researching new materials can be appreciated by considering the periodic table shown in the figure below. This table not only illustrates the relative abundance of items (the larger the box for each item, the more there is), but also highlights potential supply issues relevant to technologies that have become ubiquitous and essential in our daily lives.
Let’s take cell phones for example. All the items used in their manufacture are identified with the phone icon and you can see that many required items are becoming scarce – their future supply is uncertain. If we want to develop substitute materials that avoid the use of certain elements, the insights gained from sorting elements from their MN could prove invaluable in that research.
After 150 years, we can see that periodic tables are not only a vital educational tool, but they remain useful for researchers in their search for new essential materials. But we shouldn’t think of new versions as substitutes for previous representations. Having many different tables and lists only serves to deepen our understanding of how elements behave.
Nick Norman, professor of chemistry, Bristol University
This article was republished by The Conversation under a Creative Commons license. Read the original article.
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