The periodic table of the elements principally created by a Russian chemist, Dmitry Mendeleev (1834-1907), celebrated its 150th anniversary last year. It might be hard to overstate its importance as an organizing principle in chemistry. All budding chemists become conversant in it from the earliest stages of their education.
The table’s importance, one could be forgiven for thinking that the ordering of the elements were not subject to debate. However, 2 scientists in Moscow, Russia have recently published a proposal for a new order.
Let’s first consider how the periodic table developed. By the late 18th century, chemists were clear about the difference between a component & a compound: elements were chemically indivisible (such as hydrogen, oxygen) whereas compounds consisted of two or more elements in combination having properties quite different from their component elements.
By the early 19th century, there was good evidence for the existence of atoms. And by the 1860s, it had been possible to list the known elements according to their relative atomic mass. As an example, hydrogen was 1 & oxygen was 16.
Simple lists are of course one-dimensional in nature. But chemists were already aware that certain elements had rather similar chemical properties. For instance, lithium, sodium & potassium or chlorine, bromine & iodine.
Something appeared to repeat & by placing chemically similar elements next to each other, a 2-dimensional table might be constructed. The periodic table was created.
Importantly, Mendeleev’s periodic table was derived empirically based on the observed chemical similarities of certain elements. It might not be until the early 20th century, after the structure of the atom had been established & following the development of quantum theory that a theoretical understanding of its structure would emerge.
Elements were now ordered according to their atomic number (the number of positively charged particles called protons present in the atomic nucleus), instead of by their atomic mass, but still also by chemical similarities.
But the latter now followed from the arrangement of electrons repeating in “shells“ at regular intervals. By the 1940s, most textbooks featured a periodic table almost similar ones we see today.
It would be understandable to think that this is the end of the matter. Not so, however. A search of the web will reveal all kinds of versions of the periodic table.
There are short versions, long versions, circular versions, spiral versions & even three-dimensional versions. Many of these for sure are simply different ways of conveying the same information, but there still be disagreements about where some elements should be placed.
The precise placement of certain elements depends on which particular properties we want to highlight. Thus, a periodic table which provides primacy to the electronic structure of atoms will differ from tables that the principal criteria are certain that is chemical or physical properties.
These versions don’t differ by much, but there are certain elements, hydrogen for instance, which one might place quite differently consistent with the actual property one want to highlight. Some tables place hydrogen in group 1 whereas in others it placed at the top of group 17; some tables even have it in a group on its own.
Rather more radically, however, we might also consider ordering the elements in a very different way, one which doesn’t involve atomic number or reflect electronic structure, reverting to a one-dimensional list.
New proposal
The latest plan to order elements in this manner was recently published in the Journal of chemistry by scientists Zahed Allahyari & Artem Oganov.
Their approach, building on the earlier work of others, is to assign to each element a Mendeleev Number (MN).
There are several ways to derive such numbers but the newest study uses a mixture of two fundamental quantities which can be measured directly i.e. an element’s atomic radius & a property called electronegativity which describes how strongly an atom attracts electrons to itself.
If one orders the elements by their MN, nearest neighbours have unsurprisingly, rather similar MNs. But of more use is to require this one step further & construct a two-dimensional grid-based MN of the constituent elements in so called “binary compounds“.
These are compounds composed of two elements, like common salt, NaCl (Sodium Chloride).
What is the advantage of this approach? Importantly, it helps to predict the properties of binary compounds that haven’t been made yet. This is often useful in the look for new materials that are likely be needed for both future & existing technologies. In time, this may be extended to compounds with more than two elemental components.
Take mobile phones, for example. All the elements used in their manufacture are identified with the phone icon & you’ll see that several required elements are getting scarce, their future supply is uncertain.
If we are to develop replacement materials which avoid the utilization of certain elements, the insights gained from ordering elements by their MN may prove valuable therein search.
After 150 years, we will see that periodic tables aren’t just an important educational tool, they continue to be useful for researchers in their quest for essential new materials. But we should always not consider new versions as replacements for earlier depictions. Having many various tables & lists only serves to deepen our understanding of how elements behave.
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