Holmium – facts and properties

Position in the periodic table

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Occurrence and extraction

Holmium is one of the heavy rare earths, whose shared traits are explained on the lanthanum page. In the Earth's crust it is more than ten times as common as silver, yet it forms no deposits of its own. It sits in small shares in xenotime, monazite and the ion-adsorption clays of southern China, always together with its neighbours dysprosium and erbium. It turns up only as a by-product when these ores are processed. The metal is obtained by reducing the fluoride with calcium.

Holmium was found with a spectroscope. In 1878 the Geneva chemists Jacques-Louis Soret and Marc Delafontaine saw absorption lines in the spectrum of an erbium sample that matched no known element. A year later the Swede Per Teodor Cleve separated the new element chemically in Uppsala. He named it after Holmia, the Latin word for his home town of Stockholm. From the same sample Cleve also pulled out a second new earth, which led to thulium.

Properties and reactions

Holmium is the eleventh lanthanide. In period 6, dysprosium sits to its left and erbium to its right. The electron configuration is [Xe] 4f¹¹ 6s² and the electronegativity is 1.23. The metal has a density of 8.8 g/cm³ and an atomic radius of 176 pm. Its melting point is 1474 °C and its boiling point 2700 °C. Holmium is soft and workable and tarnishes only slowly in damp air. When heated it burns to yellowish holmium oxide. Its compounds contain it as a triply charged ion.

What sets holmium apart is its magnetism. In the atom, the spins of the unpaired 4f electrons and their orbital motion add up to a very large magnetic moment. By the usual figures it shares the largest moment of any element with dysprosium. At room temperature the metal is only paramagnetic, so its atomic magnets line up only inside an outside field. At very low temperatures it concentrates a magnetic field more strongly than any other element, which makes it interesting for pole pieces.

Its light is striking too. Ho³⁺ ions absorb light only in very narrow bands of wavelength, because the electrons involved sit deep inside the 4f subshell and are hardly disturbed by their surroundings. The salts and the oxide are yellowish, and glass containing holmium looks yellow or pink depending on the lighting.

Uses in everyday life and technology

The magnetism is put to work in pole pieces. These are parts made of holmium that squeeze the field of a coil into a small space, as very strong laboratory magnets require.

In medicine, the best-known use is the holmium YAG laser. Its yttrium aluminium garnet crystal holds a little holmium, which emits infrared light at a wavelength of about 2100 nanometres. Water absorbs this radiation very strongly, so it penetrates only a wafer-thin layer. In urology a thin glass fibre is passed through an endoscope up to the stone. The laser pulses break it into small fragments or fine dust, which the body can pass.

In the laboratory, holmium oxide serves as a reference standard. A spectrometer measures how much light a sample absorbs at which wavelength, so its wavelength scale has to read correctly. To check it, a glass or solution containing holmium oxide is placed in the beam. Its absorption bands lie at fixed, known wavelengths, for example near 360 and 540 nanometres, and any deviation in the reading reveals an error.

Quick question

What is the atomic number of holmium (Ho)?

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