- Thulium (Tm) is a silvery-gray, soft, and malleable rare earth metal with atomic number 69, belonging to the lanthanide series of the periodic table.
- Its electron configuration is [Xe] 4f¹³ 6s², and it predominantly forms Tm³⁺ ions as its most stable oxidation state, though the +2 state is also known in certain compounds. The atomic structure contains sixty-nine protons, typically one hundred or one hundred one neutrons in its natural isotope, and sixty-nine electrons arranged in six shells.
- Naturally occurring thulium is monoisotopic, consisting entirely of the stable isotope thulium-169 (¹⁶⁹Tm).
- Thulium is one of the least abundant lanthanides in the Earth’s crust, with an estimated concentration of about 0.5 parts per million. It is not found in its free metallic state in nature but occurs in small amounts within minerals such as monazite ((Ce,La,Nd,Th)PO₄) and xenotime (YPO₄). Its extraction involves ion-exchange and solvent extraction techniques to separate it from other rare earth elements. The main producers are China and, to a lesser extent, the USA, Russia, and Australia.
- Thulium was discovered in 1879 by Swedish chemist Per Teodor Cleve, who separated it from other rare earths using repeated chemical fractionation of erbium and holmium compounds. The element’s name is derived from “Thule,” a classical ancient name for the far north, often associated with Scandinavia.
- Although thulium has limited large-scale industrial use due to its rarity and cost, it has several specialized applications. Thulium-170 (¹⁷⁰Tm), a radioactive isotope produced by neutron irradiation of thulium-169, emits X-rays and is used in portable radiography devices for non-destructive testing. Thulium is also employed as a dopant in solid-state lasers, such as thulium-doped YAG lasers, which emit in the near-infrared region and are used in medical procedures, including laser surgery and lithotripsy for kidney stones. Thulium-doped optical fibers are important in certain telecommunications and high-power laser systems. Additionally, thulium oxide (Tm₂O₃) can be used in ceramic capacitors and as a component in specialized glass formulations.
- Chemically, thulium is relatively reactive for a lanthanide. It slowly tarnishes in air, forming a greenish-gray oxide layer. Thulium reacts slowly with cold water but more readily with hot water, producing thulium hydroxide and hydrogen gas. It dissolves in acids, yielding nearly colorless Tm³⁺ salts.
- Biologically, thulium has no known essential role in living systems. It is considered to have low to moderate toxicity, but, as with all rare earth compounds, inhalation or ingestion of dust and soluble salts should be avoided.
- From an environmental standpoint, thulium in its mineral form is stable and not hazardous. Environmental concerns primarily arise from rare earth mining and refining processes, which can generate chemical waste and sometimes release naturally occurring radioactive materials present in the ore.
