By the end of the 18^th century, scientists had defined "elements" as basic building blocks of nature that no chemical means could decompose. Several decades later, Mendeleev conceived his periodic table, reportedly upon waking from a dream one afternoon. Its genius lay in the order it brought to the cacophony of elements. Each one was assigned to a row in the table, which in turn corresponded to an elemental category. Mendeleev laid out in one stroke the underlying order of matter, hitherto dimly perceived at best.

The table was full of holes; Mendeleev concluded simply that chemists had failed to discover a huge number of elements. His colleagues derided this idea. Then, six years after the table's creation, the French scientist Paul-Émile Lecoq de Boisbaudran found one of the missing elements—now called gallium. Aptly named, one might say, for Mendeleev's own considerable gall. Fifty-two more elements have been discovered or created since, for a total of 116.