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Dmitri Mendeleev

8 February 1834 - 2 February 1907
Dmitri Mendeleev came up with a predictive version of the periodic table of elements

Dmitri Mendeleev

Dmitri Mendeleev came up with a predictive version of the periodic table of elements

Ria Novosti/Science Photo Library

On 17 February 1869, Russian chemist Dmitri Mendeleev jotted down the symbols for the chemical elements, putting them in order according to their atomic weights and inventing the periodic table. He wrote down the sequence in such a way that they ended up grouped on the page according to known regularities or ‘periodicities’ of behaviour. It was perhaps the greatest breakthrough in the history of chemistry.

Mendeleev’s ideas, which built on the earlier work of French chemist Antoine Lavoisier in the previous century, totally changed the way chemists viewed their discipline. Now each chemical element had its number and fixed position in the table, and from this it became possible to predict its behaviour: how it would react with other elements, what kind of compounds it would form, and what sort of physical properties it would have.

Soon, Mendeleev was predicting the properties of three elements – gallium, scandium and germanium – that had not then been discovered. So convinced was he of the soundness of his periodic law that he left gaps for these elements in his table. Within twenty years, all three had been found, and their properties confirmed his predictions almost exactly.

Mendeleev himself was surprised by how fast his ideas were confirmed. In a prestigious Faraday Lecture to the Royal Institution in London in 1889, he admitted that he had not expected to live long enough ‘to mention their discovery to the Chemical Society of Great Britain as a confirmation of the exactitude and generality of the periodic law’. As news of his remarkable accomplishment began to spread, Mendeleev became something of a hero, and interest in the periodic table soared.

In all, Mendeleev predicted 10 new elements, of which all but two turned out to exist. He later proposed that the positions of some pairs of adjacent elements be reversed to make their properties fit into the periodic pattern. He suggested swapping cobalt with nickel and argon with potassium, which he believed had been wrongly placed because their true atomic weights were different from the values chemists had determined. It took until 1913, some six years after Mendeleev had died, to clear up this ambiguity. By then chemists had gained a much better understanding of the atom, and in that year the physicist Henry Moseley, working in Manchester, showed that the position of an element in the table is governed not by its atomic weight but by its atomic number. Dennis Rouvray

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