Thulium

Thulium is a chemical element with the symbol Tm and atomic number 69. It is the thirteenth element in the lanthanide series of metals. Thulium is a rare substance found in the Earth's crust. In fact, it is the second-least abundant lanthanide. Only the radioactive element promethium is less common.

This element is valued for its unique physical and chemical properties. Despite its rarity and high cost, it plays vital roles in modern technology. It is used in medical tools and advanced laser systems.

As a metal, thulium has a bright, silvery-gray luster. It is quite soft and can be cut with a knife. Scientists measure its hardness between 2 and 3 on the Mohs scale. This makes it both malleable and ductile. Malleable means it can be hammered into sheets. Ductile means it can be drawn into wires. However, thulium reacts with its environment. It tarnishes slowly when exposed to air. If it reaches 150°C, it can burn readily to form thulium(III) oxide. Thulium dust is particularly dangerous because it can cause fires or explosions.

Thulium shows different magnetic behaviors depending on its temperature. It is ferromagnetic below 32K. This means it can be strongly attracted to magnets. Between 32K and 56K, it becomes antiferromagnetic. Above 56K, it is paramagnetic. The metal also exists in two different structural forms called allotropes. These are the tetragonal α-Tm and the more stable hexagonal β-Tm. In chemical solutions, thulium ions often form coordination complexes. In these complexes, a single thulium ion is surrounded by nine water molecules. These specific ions exhibit a bright blue luminescence.

The discovery of thulium is a story of careful scientific observation. In 1879, Swedish chemist Per Teodor Cleve identified the element.

He was studying the oxides of other rare earth elements. He found two new components in the mineral erbia. He called them holmia and thulia. Thulium was named after Thule, an ancient name for Scandinavia. It took many years to achieve high purity. Charles James obtained nearly pure thulium in 1911 using fractional crystallization. He performed 15,000 operations to ensure the material was homogeneous. Later, in 1936, Wilhelm Klemm and Heinrich Bommer obtained the pure metal.

Thulium is quite difficult to find in nature. It is never found in a pure form on its own. Instead, it exists in small amounts within various minerals. It is frequently found in monazite, xenotime, and euxenite. It also occurs in the mineral gadolinite.

Most thulium is extracted from monazite ores found in river sands. Today, much of the supply comes from ion adsorption clays in southern China. The element's abundance in the Earth's crust is only 0.5 mg/kg. In 2001, total world reserves were estimated at 100,000 tonnes. Because it is so rare, it is very expensive. In 2005, 99%-pure thulium metal powder cost approximately $70 per gram.

Modern technology relies on thulium for several specialized tasks. One major use is as a dopant in solid-state lasers. A dopant is a substance added to change a material's properties. Thulium-doped YAG lasers are highly efficient. They operate at a wavelength of 2010 nm. These lasers are useful for surgery because they can remove tissue with minimal damage. Another application is in portable X-ray devices. Scientists use the isotope thulium-170 for this purpose. This isotope has a half-life of 128.6 days. These X-ray tools are used in medicine, dentistry, and industrial inspections.

Thulium also has interesting uses in security and science. The element fluoresces with a blue color under ultraviolet light. Because of this unique glow, thulium is included in euro banknotes. This helps prevent counterfeiting by making the notes harder to fake. In biology, thulium has a low level of toxicity. It does not have a significant biological role in living things. However, soluble thulium salts can be mildly toxic if injected. In humans, the element is mostly found in the liver, kidneys, and bones. Humans typically consume only a few micrograms of thulium each year.