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 difficult to overestimate its importance as an organizing principle in chemistry – all novice chemists get acquainted with it from the earliest stages of their education.
Given the importance of the table, one can forgive the idea that the arrangement of the elements is no longer subject to debate. Two scientists from Moscow, Russia, recently published a proposal for a new order.
Let’s first look at how the periodic table is designed. Towards the end of the 18th century, chemists were aware of the difference between an element and a compound: elements are chemically inseparable (examples are hydrogen, oxygen), while compounds consist of two or more elements in combination having properties quite different from their constituents elements.
In the early 1
Simple lists, of course, are one-dimensional. But chemists were aware that some elements had very similar chemical properties: for example, lithium, sodium and potassium, or chlorine, bromine and iodine.
It seems that something is repeating and by placing chemically similar elements next to each other, a two-dimensional table can be built. The periodic table was born.
It is important that the periodic table of Mendeleev is derived empirically on the basis of the observed chemical similarities of some elements. Only at the beginning of the 20th century, after the structure of the atom was established and after the development of quantum theory, will a theoretical understanding of its structure emerge.
The elements were now arranged by atomic number (the number of positively charged particles called protons in the atomic nucleus), not by atomic mass, but by chemical similarities.
But the latter now follows from the arrangement of electrons repeating in the so-called “shells” at regular intervals. By the 1940s, most textbooks had a periodic table similar to the one we see today, as shown in the figure below.
It would be understandable to think that this would be the end of the matter. However, this is not the case. A simple internet search will reveal all versions of the periodic table.
There are short versions, long versions, circular versions, spiral versions and even three-dimensional versions. Many of them, of course, are just different ways of conveying the same information, but there are still disagreements about where some elements should be placed.
The exact location of certain elements depends on which specific properties we want to emphasize. Thus, the periodic table, which gives preference to the electronic structure of atoms, will differ from the tables for which the main criteria are certain chemical or physical properties.
These versions do not differ much, but there are certain elements – such as hydrogen – that one can place quite differently according to the specific property you want to emphasize. Some tables place hydrogen in group 1, while in others it is at the top of group 17; some tables even have it alone in a group.
However, we can think more radically about arranging the elements in a very different way, which does not include an atomic number or reflects an electronic structure – a return to a one-dimensional list.
The latest attempt to arrange items in this way was recently published in Journal of Physical Chemistry by scientists Zahed Allahyari and Artem Oganov.
Their approach, building on the earlier work of others, is to assign to each element what is called a Mendeleev number (MN).
There are several ways to extract such numbers, but the latest study uses a combination of two basic quantities that can be measured directly: the atomic radius of the element and a property called electronegativity, which describes how strongly an atom attracts electrons.
If someone arranges the elements by their MN, the nearest neighbors have, not surprisingly, quite similar MNs. But it is more useful to take this step further and build a two-dimensional network based on the MN of the constituents in the 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 made. This is useful when looking for new materials that are probably needed for both future and existing technologies. Over time, no doubt, this will be extended to compounds with more than two elementary components.
A good example of the importance of searching for new materials can be assessed by looking at the periodic table shown in the figure below.
This table illustrates not only the relative abundance of elements (the larger the box for each element, the more there are), but also highlights potential supply problems related to technologies that have become ubiquitous and important in our daily lives.
Take mobile phones, for example. All the elements used in their production are identified by the phone icon and you can see that several necessary elements become scarce – their future supply is uncertain.
If we want to develop substitute materials that avoid the use of certain elements, the insights gained from ordering elements from their MN can be valuable in this search.
After 150 years, we can see that periodic tables are not only a vital educational tool, they remain useful for researchers in their search for essential new materials. But we should not think of new versions as substitutes for earlier images. The existence of many different tables and lists only serves to deepen our understanding of the behavior of the elements.
Nick Norman, Professor of Chemistry, University of Bristol.
This article is republished by The Conversation under a Creative Commons license. Read the original article.