Holmium

Holmium is a soft, silvery lanthanide with the configuration [Xe]4f11 6s2. It forms the Ho3+ ion, whose ten 4f electrons arrange themselves to give the highest magnetic moment of any naturally occurring element, about 10.6 Bohr magnetons. Because of this, holmium is used for the pole pieces that concentrate the field in the strongest static laboratory magnets. The metal itself becomes ferromagnetic only below about 20 K.

Holmium's sharp 4f absorption bands give its compounds unusual colours. Holmium oxide looks pale yellow in daylight but pinkish under fluorescent lighting, because its narrow absorption lines interact differently with the two kinds of light. Those same sharp bands make holmium oxide solutions and glass a standard for calibrating spectrophotometer wavelengths. In medicine, holmium-doped YAG lasers emit at 2.1 micrometres, a wavelength strongly absorbed by water, so they can shatter kidney stones and cut tissue with little damage to surrounding areas.

Discovery

In 1878 Swiss chemists Marc Delafontaine and Jacques-Louis Soret noticed unknown absorption lines in erbium samples and called the source 'element X'. Working independently in 1878–79, Swedish chemist Per Teodor Cleve separated the oxide from erbia and named it holmia, from Holmia, the Latin name of his home city, Stockholm. Pure holmium oxide was later prepared, and the metal was first isolated in reasonably pure form in the 20th century.