The periodic table of elements, primarily created by the Russian chemist Dmitry Mendeleev (1834–1907), celebrated its 150th anniversary last year. Its importance as an organizing principle in chemistry will be difficult to overcome – all budding chemists become familiar with it from the early stages of their education.
Given the importance of the table, it can be forgiven to think that the order of the elements were no longer subject to debate. However, two scientists in Moscow, Russia, have recently published a proposal for a new order.
Let us first consider how the periodic table developed. By the late 18th century, chemists were clear about the difference between an element and a compound: the elements were chemically inseparable (examples are hydrogen, oxygen), while compounds consisted of a combination of two or more elements whose components. Elements have significantly different properties.
By the early 19th century, there was good circumstantial evidence for the existence of atoms. And by the 1860s, it was possible to list known elements in the order of their relative atomic masses – for example, hydrogen 1 and oxygen 16.
Simple lists are, of course, one-dimensional in nature. But chemists knew that some elements had similar chemical properties: for example lithium, sodium and potassium or chlorine, bromine and iodine.
A two-dimensional table could be constructed, with some repeating and placing chemically identical elements next to each other. The periodic table was born.
Importantly, Mendeleev’s periodic table was derived empirically based on observed chemical similarities of some elements. It would not be until the early 20th century, when the structure of an atom was established and after the development of quantum theory, that a theoretical understanding of its structure would emerge.
The elements were now ordered by atomic number (the number of positively charged particles called protons in the atomic nucleus) rather than atomic mass, but there are still chemical similarities.
But the latter is now followed at regular intervals by the arrangement of electrons in so-called “shells”. By the 1940s, most textbooks had a periodic table that we see today, as shown in the figure below.
It is reasonable to understand that this will be the end of the matter. However, it is not so. A simple search of the Internet will reveal all types 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 remains a disagreement about whether certain elements should be kept.
The exact location of some elements depends on which particular properties we want to highlight. Thus, a periodic table that gives precedence to the electronic structure of atoms will be different from tables for which the principal criteria are some chemical or physical properties.
These versions are not very different, but there are some elements – for example hydrogen – that can be differentiated according to a particular property that anyone wants to uncover. Some tables hold hydrogen in group 1 while in others it sits on top of group 17; In some tables, it also occurs in a group by itself.
Rather more fundamentally, however, we can also consider ordering elements very differently, not including atomic numbers or reflecting electronic structure – by returning to a dimensional list.
The latest attempt to order elements in this way was recently published Journal of Physical Chemistry Zahid Allah and Qari Oganov by scientists.
His approach, building on the earlier work of others, is to assign to each element what is called the Mendeleev number (MN).
There are many ways to obtain 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, which states That how an atom attracts electrons in itself.
If one orders the elements by their MNs, then the nearest neighbors have indefinitely, rather similar MNs. But for more use it has to be taken one step further and to construct a two dimensional grid based on MN of the so-called “binary compound”.
These are two elements, such as sodium chloride, NaCl.
What is the benefit of this approach? Importantly, it may help predict the properties of binary compounds that have not yet been formed. It is useful in discovering new materials that have potential for both future and existing technologies. In time, no doubt, it will be extended to compounds with more than two fundamental components.
A good example of the importance of discovering new material can be done by considering the periodic table shown in the figure below.
This table not only shows the relative abundance of elements (the larger the box for each element, the greater there is), but also highlights potential supply issues for technologies that have become ubiquitous and necessary in our daily lives.
For example, take a mobile phone. All the elements used in their manufacture are identified by the phone icon and you can see that many essential elements are becoming scarce – their future supply is uncertain.
If we are developing substitution materials that avoid the use of certain elements, the insights gained from the elements ordered by their MNs may prove valuable in that discovery.
After 150 years, we can see that periodic tables are not only an important educational tool, they remain useful to researchers in the search for the necessary new materials. But we should not think of a replacement for earlier statements about new versions. Having many different tables and lists only deepens our understanding of how elements behave.
Nick Norman, Professor of Chemistry University, Bristol.
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