Europium

Europium has the configuration [Xe]4f7 6s2, and its half-filled 4f shell explains most of its character. Because 4f7 is especially stable, europium readily stops at the Eu2+ ion after losing only its two 6s electrons, behaving more like calcium or strontium than like a typical lanthanide. Only two electrons enter its metallic bonding instead of three, so the metal is the least dense lanthanide, melts at a low 822 °C, and is the most reactive of the rare earths: it tarnishes within hours in air and must be stored under oil or argon.

Europium's ions are famous emitters of light. Eu3+ gives a sharp, saturated red line near 612 nanometres, while Eu2+ produces broad blue to green emission depending on its host crystal. In both cases the 4f electrons are shielded by outer shells, so their energy levels shift little from host to host. Phosphors such as yttrium oxide doped with europium supplied the red in colour televisions and fluorescent lamps, and Eu2+ is the activator in glow-in-the-dark strontium aluminate.

Discovery

Europium's spectral lines were noticed in samarium samples during the 1880s and 1890s, but French chemist Eugène-Anatole Demarçay was the first to separate it convincingly, isolating it in 1901 after painstaking fractional crystallisation of samarium magnesium nitrate. He named it after the continent of Europe. Pure europium metal was not produced until 1937, and large-scale separation became practical only in the 1950s with ion-exchange methods.