Dmitri Mendeleev was unsure if atoms were real. But his table of the elements reflects their structure
Gallium is an unusual element. It is a light, silvery metal, similar to aluminium. But unlike aluminium, if you hit it with a hammer it shatters like glass. It melts at about 30°C—in your hand, or just on a hot day. But what really sets it apart is that it was the first element discovered that had already been predicted to exist.Dmitri Mendeleev, a Russian chemist, devised the periodic table 150 years ago this month.
The periodic table is a familiar sight today, and its peculiar shape is now understood to reflect the way electrons arrange themselves around the nucleus inside the atom. Electrons also determine much of how elements behave, which is why the table reflects their properties so well. It was not until the advent of quantum physics in the 1920s and 1930s that chemists understood the way that energy levels and orbitals explain the periodic table. Mendeleev knew none of it: prevailing theory in the 1860s was that atoms were indivisible, and had no internal structure. Electrons had yet to be discovered and Mendeleev himself was sceptical that atoms really existed.
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