The Periodic Table of Elements, created primarily by the Russian chemist Dmitry Mendeleev (1834-1907), celebrated its 150th anniversary last year. Its importance as an organization theory of chemistry will be difficult to understand – all emerging chemists are familiar with it from an early stage of their education.
Given the importance of the table, one can be forgiven for thinking that the order of the elements is no longer a topic of discussion. However, two scientists in Moscow, Russia, have recently published a proposal for a new order.
Let us first consider how the periodic table evolved. 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 have two or more elements in a compound, with properties quite different from their constituent elements. In the early 19th century, there was evidence of good circumstances for the existence of atoms. And by the 1860s, it was possible to list known elements in the order of their respective atomic mass – for example, hydrogen 1 and oxygen 16.
The simple list is, of course, one-dimensional in nature. But chemists knew that certain elements had the same chemical properties instead: for example lithium, sodium and potassium or chlorine, bromine and iodine. Something felt repetitive and by placing chemically identical elements next to each other, a two-dimensional table could be created. The periodic table was born.
Importantly, Mendeleev’s periodic table is taken empirically based on the observed chemical similarities of the experience elements. It will not be until the beginning of the 20th century that the formation of the atom is established and following the development of quantum theory, a theoretical understanding of its formation will emerge.
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Elements were now ordered by atomic number (number of positively charged particles known as protons in the atomic nucleus), rather by atomic mass, but still by chemical similarity. But later the electrons are now arranged at regular intervals in the so-called “shells”. By the 1940s, most textbooks featured the periodic table 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 search of the Internet will reveal all sorts of versions of the periodic table. There are also shorter versions, longer versions, rounded versions, spiral versions and three-dimensional versions. Many of these, to be sure, are just different ways of conveying the same information but disagree on where some elements should be placed.
The exact placement of certain elements depends on what special properties we want to highlight. Thus, the periodic table that prefers the electronic composition of atoms may differ from the tables, for which the main criteria are certain chemical or physical properties.
These versions are not very different, but they do have some elements – for example hydrogen – that can make a person quite different depending on the property he wants to highlight. Some tables place hydrogen in group 1 while in others it sits at the top of group 17; Some tables even have it in its own group.
More radically than that, however, we can also consider ordering elements very differently, one that does not contain atomic numbers or reflects an electronic structure – returns to a one-dimensional list.
New proposal
A recent attempt to order the elements in this way was recently published in the Journal of Physical Chemistry by scientists Zniko Zehd Allahyari and Artem Oganov. His approach is to assign to each element what is called the Mendeleev number (MN), taking into account the previous work of others. There are many ways to obtain such numbers, but a recent study 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 an atom attracts electrons to itself.
If any of their M.N. Orders to the elements by, nearby neighbors have, surprisingly, the same M.N. But more use is to take this one step further and the so-called “binary combinations.” Creating a two-dimensional grid based on the MN of the constituent elements in. These are compounds made up of two elements like sodium chloride, NACL.
What is the advantage of this approach? Importantly, it can help predict the properties of binary compounds that have not yet been created. This is useful in discovering new content that is necessary for both future and existing technology. Over time, no doubt, this will be expanded into combinations with more than two components.
Given the periodic table shown periodically in the figure below, a good example of the importance of discovering new material can be appreciated. This table not only illustrates the relative abundance of elements (the BX for each element is larger, more of it) but also highlights potential supply issues related to technologies that have become ubiquitous and essential in our daily lives.
Take a mobile phone, for example. All the elements used in their production 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 to make replacement materials that avoid the use of certain elements, their M.N. The insights gained from ordering elements by can be valuable in that search.
150 years later, we can see that periodic tables are not only an important educational tool, they remain useful for researchers in search of essential new material. But we should not think of new versions as a substitute for previous pictures. Having multiple tables and lists enhances our understanding of how elements behave.
This talk has been republished by Professor Nick Norman of the University of Bristol under the Creative Commons license. Read the original article.
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