The periodic table of elements, mainly created by the Russian chemist Dmitry Mendeleev (1834-1907), celebrated its 150th anniversary last year. Its importance as a principle of organizing chemistry cannot be overestimated – all aspiring chemists will learn it from the earliest stages of their education.
Given the importance of the table, one might be forgiven for thinking that the order of the items was no longer discussed. However, two scientists from Moscow in Russia recently published a proposal for a new order.
Let’s first consider how the periodic table was developed. At the end of the 18th century, chemists clearly defined the difference between an element and a compound: the elements were chemically indivisible (e.g., hydrogen, oxygen), while the compounds consisted of a combination of two or more elements and had properties quite different from their constituent elements.
In the early nineteenth century, there were good indications of the existence of atoms. In the nineteenth century, it was possible to list the known elements in order of their relative atomic mass – for example, hydrogen was 1 and oxygen was 16.
Of course, simple lists are one-dimensional. But chemists were aware that certain elements had fairly similar chemical properties: for example, lithium, sodium and potassium, or chlorine, bromine and iodine.
It seemed that something was repeating itself, and by placing chemically similar elements next to each other, it was possible to build a two-dimensional table. The periodic table of elements is born.
Importantly, Mendeleev’s periodic table was derived empirically on the basis of the observed chemical similarities of some elements. Only at the beginning of the 20th century, after establishing the structure of the atom and developing quantum theory, will the theoretical understanding of its structure emerge.
The elements were now ordered by atomic number (the number of positively charged particles called protons in the atomic nucleus), not by atomic mass, but still by chemical similarities.
But the latter was now due to the arrangement of the electrons repeating in so-called “shells” at regular intervals. In the 1940s, most textbooks contained a periodic table similar to what we see today as shown in the figure below.
It would be understandable to think that this is over. However not so. A simple internet search will reveal different versions of the periodic table.
There are short, long, round, spiral and even three-dimensional versions. Of course, many of them are just different ways of communicating the same information, but there are still disagreements as to where to put certain elements.
The exact arrangement of individual elements depends on which properties we want to emphasize. Thus, the periodic table that gives priority to the electronic structure of atoms will differ from tables for which specific chemical or physical properties are the main criteria.
These versions aren’t that different, but there are certain elements – hydrogen, for example – that can be placed very differently depending on the specific property we want to highlight. Some tables put hydrogen in group 1, while in others it is at the top of group 17; on some tables she is even independent in a group.
However, more radically, we can consider ordering the elements in a completely different way that does not include atomic number or reflect electronic structure – back to the one-dimensional list.
A new proposal
A recent attempt to organize items this way was recently posted in Journal of Physical Chemistry by scientists Zahed Allahyari and Artem Oganov.
Their approach, building on previous work by others, is to assign to each element something called the Mendeleev Number (MN).
There are several ways to calculate such numbers, but recent research uses a combination of two basic 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 to itself.
If someone orders items according to their MN, the closest neighbors have, not surprisingly, rather similar MN. However, it is more useful to go a step further and construct a two-dimensional grid based on the MN of the constituent elements in so-called “binary relationships”.
They are compounds composed of two elements such as sodium chloride and NaCl.
What are the advantages of this approach? Importantly, it can help predict properties of binary relationships that have yet to be created. This is useful in finding new materials that are likely to be needed for both future and existing technologies. Over time, this will no doubt extend to compounds containing more than two elementary elements.
A good example of the importance of the search for new materials can be appreciated considering the periodic table shown in the figure below.
This table not only illustrates the relative abundance of elements (the larger the box for each item, the more there are) but also highlights potential technology-relevant supply problems that have become ubiquitous and essential in our daily lives.
Take cell phones, for example. All the components used in their production are marked with a telephone icon and you can see that there are fewer and fewer needed components – their future supply is uncertain.
If we are to develop substitute materials that avoid the use of certain items, insights gained by their MN when ordering items may prove valuable in this search.
After 150 years, we see that periodic tables are not only an essential educational tool, but remain useful for researchers in their search for new, essential materials. However, we should not think of the new versions as replacements for the earlier shows. Having many different tables and lists only serves to deepen our understanding of the behavior of the items.
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.