The periodic table of the elements, created primarily by Russian chemist Dmitry Mendeleev (1834-1907), celebrated its 150th anniversary last year. Its importance as an organizing principle in chemistry would be hard to overstate – all budding chemists become familiar with it from the earliest stages of their education.
Given the importance of the table, one can be forgiven for thinking that the order of the elements was no longer in dispute. However, two scientists in Moscow, Russia, have recently announced a proposal for a new order.
First, let’s 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: elements were chemically indivisible (examples are hydrogen, oxygen) while compounds were made up of two or more elements together, all having properties different from their components.
By the early 19th century, there was good circumstantial evidence for the existence of atoms. And by the 1860s, it was possible to list the known elements in order of relative atomic mass – for example, hydrogen was 1 and oxygen 16.
Simple lists, of course, are one-dimensional in nature. But chemists were aware that some elements have quite similar chemical properties: for example lithium, sodium and potassium or chlorine, bromine and iodine.
Something seemed to be repeating and by placing similar chemical elements next to each other, a two-dimensional board could be built. The periodic board was born.
Importantly, Mendeleev’s periodic table was derived empirically based on the observed chemical similarity of some elements. It was not until the early 20th century, after the structure of the atom had been established and the development of quantum theory, that a theoretical understanding of its structure would emerge.
Elements are now ordered by atomic number (the number of positively charged particles called protons in the atomic nucleus), not by atomic mass, but also by chemical similarity.
But the latter now followed the arrangement of electrons repeating in so-called “shells” on a regular basis. By the 1940s, most textbooks had a periodic table similar to those we see today, as shown in the figure below.
It would be understandable to think that this would be the end of the matter. Not so, though. A simple internet search will reveal all types of versions of the periodic table.
There are short versions, long versions, round versions, spiral versions and even three-dimensional versions. Many of these, to be sure, are different ways of communicating the same information but there are still disagreements about where certain elements should be placed.
The exact location of some elements depends on which specific property we want to highlight. Therefore, a periodic table that gives primacy to the electronic structure of atoms will differ from tables for which the main criteria are specific chemical or physical properties.
These versions are not much different, but there are some elements – hydrogen for example – that one could set very differently depending on the specific properties that one wants to highlight. Some tables place hydrogen in group 1 but in others it sits at the top of group 17; some boards even have a group on its own.
Rather more radically, however, we can also consider ordering the elements in a very different way, one that does not contain an atomic number or reflect an electronic structure – returning to a one-dimensional list.
The latest attempt to order elements in this way was recently announced in the Journal of Physical Chemistry by scientists Zahed Allahyari and Artem Oganov.
Their approach, building on other earlier work, is to assign to each element the so-called Mendeleev (MN) number.
There are many ways to derive such numbers, but the latest study uses a combination of two basic quantities that can be directly measured: the atomic radius of an element and a property called electronegativity that describes how strong it is. atom attracts electrons to itself.
If one orders the elements from their MNs, it is not surprising that nearest neighbors have similar MNs. But of greater use is to take this one step further and build a two-dimensional grid based on the MN of the constituent elements in what are known as “binary compounds”.
These are compounds that contain two components, such as sodium chloride, NaCl.
What is the benefit of this method? 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 likely to be needed for future and current technologies. This will undoubtedly be extended to compounds with more than two elemental components.
A good example of the importance of searching for new materials can be appreciated by considering the periodic table shown in the figure below.
This table shows not only the relative abundance of the elements (the larger the box for each element, the more there is) but also highlights potential supply issues relevant to emerging technologies. become ubiquitous and vital in our daily lives.
Take mobile phones, for example. All the components used in their production are marked with the phone icon and you can see that several required elements are running out – their future supply is uncertain.
If we are to develop new materials that avoid using certain elements, the insights gained from ordering elements from their MN can be valuable in that search.
After 150 years, we can see that periodic tables are not just an essential educational tool, they continue to be useful to researchers as they search for essential new materials. But we should not think of new versions as an alternative to earlier illustrations. Having lots of different tables and lists only serves to deepen our understanding of how elements behave.
Nick Norman, Professor of Chemistry, University of Bristol.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.