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. It assigned positions to the 63 elements known in his day, arranged in order of increasing atomic weight and aligned by similar chemical characteristics. To make it work, he had to leave gaps—missing elements—and in 1871 he used the patterns of the table to predict the properties of the elements that might fill some of them. Gallium was discovered in 1875, scandium in 1879, and germanium in 1886. All three matched Mendeleev’s predictions.

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. But by visualising the properties of the elements he knew of, he snuck a peek inside the atom and revealed a few of its secrets decades before physicists could account for them.