Lanthanum

Lanthanum is the first of the anomalies that make the f-block worth studying: despite heading the 4f series it has no 4f electron at all, filling 5d first, which is why some periodic tables put it in group 3 and leave the f-block starting at cerium. Carl Mosander found it hidden inside cerium nitrate in 1839 and named it from Greek lanthanein, to lie hidden, which was an accurate description of the next century of rare-earth chemistry. The metal is not rare — it is more abundant in the crust than lead — and its biggest single use is the LaNi₅ alloy in nickel-metal-hydride cells, the batteries that powered every early Toyota Prius and still run most hybrid vehicles.