Human beings love to categorize things; from philatelists to lepidopterists, there can be a great sense of completeness by coming up with a system detailing and organizing all the individual pieces. The chemical world is no different, and attempts to organise what we now call chemical elements have been going on since ancient times. It would, however, be the work of Dmitri Mendeleev, born 182 years ago in a Siberian village in the Russian Empire, that would give us a lasting system to categorize, understand and even predict the elements—what we now call the periodic table.
Patterns had been noticed in chemical substances from the days of the ancient Greeks, through the works of medieval alchemists and into the age of enlightenment, but one of the first recognizable efforts at a table of elements was in the first modern chemistry textbooks, Traité Élémentaire de Chimie by the “father of modern chemistry,” Antoine Lavoisier. Lavoiser created a list of substances that he believed could not be broken down further, and included substances such as zinc, sulphur and oxygen that we now know to be chemical elements, as well as ‘light’ and ‘caloric’, which we don’t. Nevertheless, by listing supposed “simple substances” and categorizing them as metals and non-metals, Lavoisier had started to pave the way.
Mendeleev used his table to predict an element one row down from silicon… The properties he predicted would eventually be proved to be incredibly accurate, and we now know this substance as “germanium.”
The 19th century saw chemists pick out further groups and similarities among the elements, including Johann Wolfgang Döbereiner’s “Triads” (1817) and John Newland’s “Law of Octaves” (1864), which posited there was some connection to the number eight in terms of chemical properties (certain similar pairs of elements had differences in mass that were multiples of eight). Ridiculed at the time for likening this to musical scales, he was eventually recognized as having realized the importance of elementary groups, and coining the term ‘periodic’ to describe elementary properties.
It was, however, in 1869 that Mendeleev made a formal presentation, and then publication, of his own work on this subject, and presented what we would now call a periodic table of elements. All great scientific discoveries have a good “discovery story” behind them and this was no exception: Mendeleev, a keen card player, was said to have used a pack of cards he had fashioned with all 63 of the then-known elements and their attributes to play games of solitaire on long train journeys. This led him to visualize the elements as belonging to various “suits” (groups) and possessing values (rows) and would share properties based on these.
Further refined in 1871, what separated Mendeleev’s table from those of his contemporaries was that it allowed him to accurately predict the existence and properties of unknown elements. For example, Mendeleev used his table to predict an element one row down from silicon. He denoted this with Sanskrit digit for “1” (“eka”) and thus named the element “ekasilicon.” The properties he predicted would eventually be proved to be incredibly accurate, and we now know this substance as “germanium.” This method is still more or less in use today, and the confirmation at the start of this year of the predicted four new row-seven elements shows how the concepts Mendeleev put to paper almost a century and half ago are being applied to elements far beyond the knowledge of his day.
Google’s “doodle” celebrating the 182nd anniversary of the birth of Dmitri Mendeleev