Earth, Water, Air, Fire. Two and a half millennia ago, Aristotle had thus solved the problem of cataloging all known substances: they were only combinations, in different proportions, of these four fundamental elements. This theory survives happily until the fifteenth century, when Paracelsus opposed it with the theory of the first tria. According to the father of alchemists, the fundamental elements from which all the others derive are only three. You will never guess them: they are Salt, Sulfur and Mercury.
We have to wait until 1661, when Robert Boyle – in his book The Skeptical Chemist – demonstrates the experimental inconsistency of previous theories and hypothesizes that substances are formed by particles that differ in size, shape, arrangement and movement. Another two centuries pass of tumultuous challenges between scientists – who attack each other with the blows of theories and defend themselves with the blows of experiments.
In 1864 we arrive at Lothar Meyer, who manages to order 44 of the 57 elements known at the time in order of valence. Only a year passed and in 1865 John Newlands proposed to catalog them in order of increasing atomic weight, noting a curious periodicity: it seems that the chemical-physical properties of the known elements are repeated more or less in groups of eight, just like the musical octaves. . Mystery…
Unfortunately, however, the proposals of Meyer and Newlands do not allow either the correct classification on the basis of the properties of the known atoms, or the prediction of new yet undiscovered elements. And now we take another step forward of only five years and we arrive on a cold Wednesday morning February 17, 1869. We are in an apartment near the University of St. Petersburg. That morning, Professor Dmitry Ivanovic Mendeleev does not have to give lessons but plans to visit dairies to study how to improve the fermentation processes that lead the milk to turn into cheese. But with the storm blowing outside the windows … let’s just say he doesn’t want to go around the frozen countryside …
To find a good reason to stay warm, he pulls out an old project of his: to look for an arrangement of the known elements based on their atomic weight and their value. Already at breakfast he scribbles some combinations of elements on the back of a newly received letter. We know for sure because this sheet – like all the other sheets used that day – is kept in his studio at the University of St. Petersburg and still shows a circular patacca left by the tea glass …
Our man continues with numerous attempts and – having found a perfect excuse to give up visits to the dairies – adds wood to the stove and decides to concentrate on his systematic work. He writes on a lot of papers trying to find a common logic by multiplying or dividing the atomic weights by the valences, trying to find common multiples that explain the differences in the atomic weights and, in short, tries a lot of combinations. Being passionate about card solitaires, he then comes up with the idea of ​​writing the name, atomic weight and valence of an element on a sheet, writing the properties of another on another sheet and going on like this until he obtains 63 tiles, each with one of the 63 elements then known.
Then try to arrange them on the table in a logical way, just like you do with the most common card solitaires. Hours go by without results… And at sunset the professor is exhausted: he decides to go for a nap. After having spent the whole day trying to understand the logic that lies behind the properties of the elements, as soon as he falls asleep what he dreams
But of course all the pieces of him. He will tell his friend Aleksandr Aleksandrovich Inostrantsevof this dream – but it is better if we call it a nightmare – in which the tiles swirl in his head. Then suddenly he wakes up, and runs to the table where he had left the pieces of the elements scattered. A few more feverish rounds of cards and this time the “solitaire” comes at the first blow: on the evening of February 17, 1869 the Periodic Table was born.
Along the lines there are the “groups” that contain elements with similar chemical properties, along the columns the “periods”, which line up the elements ordered by increasing atomic weight. But the Russian scientist has the intuition to insert empty spaces inside the table where in the future the elements that are not yet known will be placed; thus giving his table a real power of prediction of future discoveries. In 1869, only 63 elements were known. Today we have reached 118. There are those who say that we have already discovered them all and there are those who claim that there are many others still to be summarized, but this is another story.
By focusing on the holes in his table, Mendeleev was able to predict the approximate atomic weight and properties of these unknown elements. And when these were discovered or artificially produced and their properties measured, it was found that he had got it right, or at least had come very close. For example, the empty space after Aluminum 13, which Mendeleev provisionally names eka-Aluminum of possible atomic weight 68, was then occupied by Gallium 31 (atomic weight 69.7) discovered by Paul Emile Lecoq in 1875.
Even when the noble gases were discovered, it was enough to add a group to the bottom of the table and everything fit together correctly again. On that cold day that did not entice you to go around dairies, closed in his room Dmitrij Ivanovic Mendeleev invented a very effective theoretical tool that not only overturns the knowledge of the moment and catalogs the elements known until then, but above all allows us to make predictions . This is why the historian of science John D. Bernal called him “the Copernicus of chemistry”.

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