Scandium

In 1871 Mendeleev left a gap below boron in his table and described the missing element's atomic weight, oxide formula and density in advance. In 1879 Lars Fredrik Nilson, working through rare Scandinavian minerals in Uppsala, isolated an oxide that matched, and named it scandium; along with gallium and germanium it turned the periodic table from a filing system into a predictive theory. Scandium is not actually rare in the crust — it is more common than lead — but it almost never concentrates into an ore, so world production runs to a few tonnes a year and the price behaves accordingly. A fraction of a percent of scandium in aluminium refines the grain and roughly doubles the strength, which is why Al-Sc alloys appear in MiG airframes, high-end bicycle frames and 3D-printed aerospace parts.