- Holmium (Ho) is a silvery-white, relatively soft, and malleable rare earth metal with atomic number 67, belonging to the lanthanide series of the periodic table.
- Its electron configuration is [Xe] 4f¹¹ 6s², and it most commonly forms Ho³⁺ ions as its stable oxidation state. The atomic structure contains sixty-seven protons, typically ninety-eight or ninety-nine neutrons in its natural isotopes, and sixty-seven electrons arranged in six shells.
- Naturally occurring holmium is monoisotopic, composed entirely of holmium-165 (¹⁶⁵Ho), which is stable and non-radioactive.
- Holmium is not found in elemental form in nature but is present in minerals containing other rare earth elements, such as monazite ((Ce,La,Nd,Th)PO₄), xenotime (YPO₄), and gadolinite ((Ce,La,Nd,Y)₂FeBe₂Si₂O₁₀). Commercial extraction involves separating holmium from other lanthanides using ion-exchange or solvent extraction methods. The primary sources of holmium are in China, with smaller production from countries like the USA, Australia, Brazil, and India.
- The element was discovered in 1878 independently by Swiss chemists Marc Delafontaine and Jacques-Louis Soret, who first observed its distinctive absorption spectrum, and later that year by Swedish chemist Per Teodor Cleve, who isolated it from erbium oxide. Holmium is named after “Holmia,” the Latin name for Stockholm, Sweden.
- Holmium has the highest magnetic moment of any naturally occurring element, making it particularly valuable in specialized magnetic applications. It is used in alloys with dysprosium and iron for high-strength permanent magnets and magnetic pole pieces in scientific instruments. Holmium also plays a role in nuclear control rods due to its high thermal neutron absorption cross-section. In optics, holmium-doped yttrium aluminum garnet (Ho:YAG) lasers emit in the infrared region and are widely used in medical surgery, particularly for precise cutting and tissue ablation. Additionally, holmium is used in glass coloring, giving glass and cubic zirconia a yellow or pink hue.
- Chemically, holmium is relatively reactive for a lanthanide. It slowly oxidizes in dry air but more rapidly in moist air, forming a yellowish oxide layer. Holmium reacts slowly with cold water and more readily with hot water to form holmium hydroxide and hydrogen gas. It also dissolves in acids to produce pale yellow Ho³⁺ salts.
- Biologically, holmium has no known essential function in living organisms. It is considered to have low to moderate toxicity, but handling in powder or dust form should be done with care to prevent inhalation or ingestion.
- From an environmental standpoint, holmium in its natural mineral form is stable and not hazardous. However, environmental concerns can arise from the chemical processes used in rare earth mining and refinement, which may involve the release of toxic or radioactive by-products.
