Scientists propose new way of ordering the elements

The periodic table of the elements, principally created by the Russian chemist, Dmitry Mendeleev (1834-1907), celebrated its 150th anniversary last year. It would be hard to overstate its importance as an organizing principle in chemistry – all budding chemists become familiar with it from the earliest stages of their education.

Given the table’s importance, one might be forgiven for thinking that the ordering of the elements was no longer subject to debate. However, two scientists in Moscow, Russia, have recently published a proposal for a new order.

Let’s first consider how the periodic table was developed. By the late 18th century, chemists were clear about the difference between an element and a compound: elements were chemically indivisible (examples are hydrogen, oxygen) whereas compounds consisted of two or more elements in combination, having properties quite distinct from their component elements. 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 their 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 certain elements had rather similar chemical properties: for example lithium, sodium and potassium or chlorine, bromine, and iodine. Something seemed to repeat and by placing chemically similar elements next to each other, a two-dimensional table could be constructed. The periodic table was born.

Importantly, Mendeleev’s periodic table had been derived empirically based on the observed chemical similarities of certain elements. It would not be until the early 20th century after the structure of the atom had been established and following the development of quantum theory, that a theoretical understanding of its structure would emerge.

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Elements were now ordered by atomic number (the number of positively charged particles called protons in the atomic nucleus), rather than by atomic mass, but still also by chemical similarities. But the latter now followed from the arrangement of electrons repeating in so-called “shells” at regular intervals. By the 1940s, most textbooks featured a periodic table similar to the ones we see today, as shown in the figure below.