The element discovered in 1875 that confirmed Mendeleev’s prediction of eka-aluminum was gallium. French chemist Paul-Émile Lecoq de Boisbaudran identified it spectroscopically and later isolated the metal.
Mendeleev had predicted eka-aluminum in 1871, naming it for its position one place below aluminum in his table. He estimated that it would have an atomic mass of about 68 and predicted several properties, including its density, oxide, and chloride. Gallium’s measured properties were remarkably close to those forecasts.
Gallium has atomic number 31 and an atomic mass of about 69.72. It is a soft, silvery metal with an unusually low melting point of about 29.76 °C, so a piece can melt in a warm hand while remaining solid at ordinary room temperature. It is not normally found as a free element in nature.
The discovery was an important test of Mendeleev’s periodic system. Later discoveries of scandium and germanium provided further confirmation of other predictions. The names “eka-aluminum” and “gallium” therefore refer to the predicted placeholder and the element’s eventual identity, respectively.