Mendeleev’s Genius

What then did Dimitri Ivanovich Mendeleev do that sets his table apart from those earlier tables? The height of his achievement can partly be judged by the depths from which he started. Born in Tobolsk in Western Siberia in 1834, the youngest of 14 children, whose father became blind and died of tuberculosis the year Dimitri finished school, Dimitri was his mother’s favourite and she did all she could to further his education. After graduating from the Central Pedagogic Institute of St Petersburg, Dimitri went on to do research in Paris and Heidelberg for two years, before returning in 1861 to St Petersburg, where he eventually became professor of general chemistry in 1867.

He began writing a textbook of inorganic chemistry, 'Principles of Chemistry', which eventually ran into many editions and translations. In organising the material for this work, he grouped elements into chapters according to their valency. While in Germany, Mendeleev had learned of Cannizzaro’s atomic weights, and he used these to arrange the elements in ascending order.

The fateful day for Mendeleev was 17 February 1869 (Julian calendar). He cancelled a planned visit to a factory and stayed at home working on the problem of how to arrange the chemical elements in a systematic way. To aid him in this endeavor he wrote each element and its chief properties on a separate card and began to lay these out in various patterns. Eventually he achieved a layout that suited him and copied it down on paper. Later that same day he decided a better arrangement was possible and made a copy of that, which had similar elements grouped in vertical columns, unlike his first table, which grouped them horizontally. These historic documents still exist.

That Mendeleev realised that he had discovered, rather than designed, the periodic table is shown by his attitude towards it. First, he left gaps in it for missing elements. Leaving such gaps in tables of elements was not in itself new, but Mendeleev was so sure of himself that he was prepared to predict the physical and chemical properties of these undiscovered elements. His most notable successes were with eka aluminium (= Gallium) and eka-silicon (= germanium). Lecoq de Boisbaudran discovered gallium in 1875 and reported its density as 4.7g cm -3, which did not agree with Mendeleev’s prediction of 5.9g cm -3. When he was told that his new element was Mendeleev’s eka-aluminium, and had most of its properties foretold accurately, Boisbaudran redetermined its density more accurately and found it to be as predicted, 5.956 g cm -3. There could be no doubt now that Mendeleev had discovered a fundamental pattern of Nature.

Secondly, Mendeleev was prepared to place elements in his table in apparently the wrong group. Thus the oxide of beryllium had been reported to be Be2O3 by none other than the great chemist Berzelius. Later workers claimed it to be BeO. The former gave the element a valency of III, the latter II. Mendeleev had a vacancy in his table for an element in group II, and so he had no hesitation in placing beryllium in it.

Thirdly, Mendeleev was prepared to place elements in his table in the wrong order of atomic weight. The anomaly here was that tellurium (atomic weight 128) should come after iodine (127), whereas the group for Te is clearly the one before I. Mendeleev presumed that the atomic weight of Te had been determined wrongly. However, fresh analyses confirmed the original value and this anomaly remained as a puzzle for chemists until the discovery of isotopes. Where I has only a single isotope of mass number 127, Te has eight stable isotopes of mass numbers 120 to 130, and the most abundant is 130Te (32%). This results in the high average atomic weight of 128.

© John Emsley

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