Holmium

Holmium is a chemical element with the symbol Ho and atomic number 67. It belongs to the rare-earth elements and sits as the eleventh member in the lanthanide series. This metal is relatively soft, silvery, fairly resistant to corrosion, and malleable. Like most lanthanides, holmium is too reactive to occur naturally in its pure form; when exposed to air, it slowly develops a yellowish oxide coating. In dry air at room temperature, isolated holmium is fairly stable, but it reacts with water, corrodes easily, and will burn in air if heated.

In nature, holmium is found alongside other rare-earth metals such as thulium. It is a relatively scarce lanthanide, making up about 1.4 parts per million of the Earth’s crust—similar in abundance to tungsten. Swedish chemist Per Theodor Cleve first isolated holmium, while Jacques-Louis Soret and Marc Delafontaine independently observed it spectroscopically in 1878. Cleve later isolated its oxide from rare-earth ores that same year. The element’s name comes from Holmia, the Latin name for Stockholm.

Holmium occurs in minerals like monazite and gadolinite and is typically extracted from monazite using ion-exchange methods. In nature and in most laboratory chemistry, its compounds are trivalent, containing Ho(III) ions. These trivalent ions have fluorescent properties similar to other rare-earth ions, with their own unique set of emission lines, so they are used in lasers and as glass colorants.

Holmium has the highest magnetic permeability and magnetic saturation of any element, which makes it useful for the pole pieces of the strongest static magnets. Because it strongly absorbs neutrons, it is also employed as a burnable poison in nuclear reactors.

**Properties**

Holmium is the eleventh lanthanide. On the periodic table, it sits in period 6, between dysprosium on its left and erbium on its right, and above the actinide einsteinium.

**Physical properties**

With a boiling point of 3,000 K (2,727 °C; 4,940 °F), holmium is the sixth most volatile lanthanide, after ytterbium, europium, samarium, thulium, and dysprosium. At standard temperature and pressure, it normally has a hexagonally close-packed (hcp) structure, like many second-half lanthanides. Its 67 electrons are arranged as [Xe] 4f¹¹ 6s², giving it thirteen valence electrons in the 4f and 6s subshells.

Like all lanthanides, holmium is paramagnetic at standard temperature and pressure, but it becomes ferromagnetic below 19 K (−254.2 °C; −425.5 °F). It has the highest magnetic moment (10.6 μB) of any naturally occurring element and other unusual magnetic properties. When combined with yttrium, it forms highly magnetic compounds.

**Chemical properties**

Holmium metal tarnishes slowly in air, forming a yellowish oxide layer similar in appearance to iron rust. It burns readily to produce holmium(III) oxide: 4 Ho + 3 O₂ → 2 Ho₂O₃. It is relatively soft, malleable, fairly corrosion-resistant, and chemically stable in dry air at standard temperature and pressure. In moist air or at higher temperatures, it quickly oxidizes, forming a yellowish oxide. Pure holmium has a metallic, bright silvery luster.

Holmium is quite electropositive, with a Pauling electronegativity of 1.23. It is generally trivalent. It reacts slowly with cold water and quickly with hot water to form holmium(III) hydroxide: 2 Ho(s) + 6 H₂O(l) → 2 Ho(OH)₃(aq) + 3 H₂(g). Holmium metal reacts with all stable halogens, producing pink holmium(III) fluoride, yellow holmium(III) chloride, yellow holmium(III) bromide, and yellow holmium(III) iodide. It dissolves readily in dilute sulfuric acid, forming solutions with yellow Ho(III) ions that exist as [Ho(H₂O)₉]³⁺ complexes: 2 Ho(s) + 3 H₂SO₄(aq) → 2 Ho³⁺(aq) + 3 SO₄²⁻(aq) + 3 H₂(g).

**Oxidation states**

Like many lanthanides, holmium is usually found in the +3 oxidation state, forming compounds such as holmium(III) fluoride and holmium(III) chloride. In solution, it exists as Ho³⁺ surrounded by nine water molecules. Holmium dissolves in acids. It can also exist in +2, +1, and 0 oxidation states.

**Isotopes**

Natural holmium consists of one primordial isotope, holmium-165. It is observationally stable, though it should theoretically undergo alpha decay to terbium-161 with a very long half-life. Known isotopes range from ¹⁴⁰Ho to ¹⁷⁵Ho. Before stable ¹⁶⁵Ho, the primary decay mode is beta plus decay to dysprosium isotopes; after it, beta minus decay to erbium isotopes. Among the 35 synthetic radioactive isotopes, the most stable is holmium-163 (¹⁶³Ho), with a half-life of 4,570 years. Next is holmium-166 (¹⁶⁶Ho), with a half-life of 26.812 hours; others have half-lives under 4 hours. The metastable isomer ¹⁶⁶m¹Ho has an unusually long half-life of 1,133 years. With a very low excitation energy, it decays directly via beta decay rather than to the ground state, producing a particularly rich spectrum of gamma rays, making it useful for calibrating gamma-ray spectrometers. Holmium-166 (ground state) has been studied for medical applications.

**Compounds**

**Oxides and chalcogenides**

Holmium(III) oxide is the only oxide of holmium. Its color changes depending on lighting: in daylight it appears yellowish, but under trichromatic light it looks orange-red, nearly indistinguishable from erbium oxide under the same conditions.