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
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...