Which element was predicted by Mendeleev as eka-aluminium?

Gallium was the element predicted by Dmitri Mendeleev as eka-aluminium.

In 1871, Mendeleev used gaps in his periodic table to predict an undiscovered element beneath aluminium. He called it eka-aluminium, using “eka,” meaning “one” in Sanskrit, to indicate that it occupied the position one period below aluminium. His prediction included an approximate atomic mass, density, melting behavior, and the formulas of likely compounds.

French chemist Paul-Émile Lecoq de Boisbaudran discovered gallium in 1875 by examining the characteristic violet lines in the spectrum of a zinc ore. Gallium’s measured properties closely matched Mendeleev’s forecast: its atomic mass is about 69.7, its density is about 5.9 grams per cubic centimeter, and it melts at only 29.76 °C. The agreement was a major early triumph for the periodic table.

The name gallium comes from “Gallia,” the Latin name for Gaul, associated with France. Gallium is sometimes confused with germanium, Mendeleev’s eka-silicon, or scandium, his eka-boron. Today, gallium is important in compounds such as gallium arsenide, used in semiconductors, LEDs, and some solar cells.