Gadolinium

Gadolinium sits in the middle of the lanthanide row with the configuration [Xe]4f7 5d1 6s2. Losing three electrons gives Gd3+, which keeps a half-filled 4f shell with seven unpaired electrons, the largest number any ion can have. That gives gadolinium an unusually large magnetic moment. The metal is ferromagnetic below about 20 °C, close to room temperature, so a piece of gadolinium cooled in a fridge will stick to a magnet and then let go as it warms up.

In the clinic, those seven unpaired electrons speed up the relaxation of nearby water protons, which brightens tissues in magnetic resonance imaging. Free Gd3+ is toxic because it is about the same size as Ca2+ and interferes with calcium-dependent processes, so it is always given tightly wrapped in a chelating molecule. Gadolinium-157 also captures slow neutrons more strongly than any other stable nuclide, making the element valuable for reactor control and neutron detection.

Discovery

Swiss chemist Jean Charles Galissard de Marignac detected the spectral lines of a new element in 1880 in samples derived from the minerals gadolinite and samarskite. In 1886 Paul-Émile Lecoq de Boisbaudran isolated its oxide and named it gadolinium after gadolinite, which itself honours the Finnish chemist Johan Gadolin, a pioneer of rare-earth research. Pure metallic gadolinium was first prepared in 1935 by Félix Trombe.