Thulium (Tm)

• Element Symbol: Tm
• Atomic Number: 69
• Atomic Mass: 168.9342
• Group # in Periodic Table: n/a
• Group Name: Lanthanides
• Period in Periodic Table: 6
• Block of Periodic Table: f-block
• Discovered by: Per Teodor Cleve (1879)

Thulium is a metallic chemical element of the periodic table. It is a rare earth metal belonging to the lanthanide group of elements, which includes fourteen other rare earth elements, such as lanthanum, yttrium, and promethium. Rare earth elements typically occur together in nature and are oftentimes difficult to separate.

The first rare earth elements began to be discovered in the late 1700s, when Johan Gadolin separated the element yttrium from the mineral gadolinite. Scientists soon realized that samples of rare earth metals from such minerals often were contaminated with additional elements, and in 1843 Carl Mosander successfully extracted from yttrium two additional rare earths, erbium and terbium. It was by following this same strategy that in 1879 Swedish chemist Per Teodor Cleve discovered the element thulium at the University of Uppsala in Sweden. Cleve began experimenting with erbium oxide, removing all of its previously known impurities. Upon further reduction, Cleve extracted two new materials, one brown and one green. The brown material he named "holmia," the oxide of the element holmium, and the green material became "thulia," the oxide of the element thulium. The physical and chemical properties of thulium were difficult to study until 1911, when Charles James prepared the first pure thulium sample using a process that would later be called the James method in his honor.

Physical Properties

Thulium has a bright silvery-gray metallic color. At 298 kelvins (K), thulium’s standard state is a ductile solid, with a density of 9.32 grams per cubic centimeter (g/cm³). In this state, thulium is soft and malleable, meaning it can be cut with a knife; it is a 2 on the Mohs hardness scale. Thulium is somewhat stable and more resistant to oxidation than most rare earth metals, but it slowly begins to oxidize at room temperature. The melting point of thulium is 1545 degrees Celsius (°C). Its boiling point is 1950 °C. The specific heat of thulium at 298 K is 160 joules per kilogram-kelvin (J/kg·K). Thulium is a good conductor, with an electrical conductivity of 1.4 × 10⁶ siemens per meter (S/m). Its thermal conductivity is 17 watts per meter-kelvin (W/m·K). At temperatures below −241.15 °C, thulium is considered ferromagnetic (the strongest type of magnetism); at temperatures between −241.15 °C and −217.15 °C, it becomes weaker and antiferromagnetic; and at temperatures above −217.15 °C, it becomes paramagnetic (only magnetized when placed within a magnetic field).

Chemical Properties

Common oxidation states of thulium are +3 and +2. Thulium reacts slowly with cold water, but it reacts quickly with hot water. It also reacts slowly with all halogens at 24.85 °C (or 298 K), but it reacts quite quickly when temperatures exceed 199.85 °C. Most thulium compounds are green in color, although thulium ions glow a bright blue when excited. Thulium has a hexagonal close-packed crystal structure.

Naturally occurring thulium consists solely of thulium-169, the element’s only stable isotope. In addition to this stable isotope, thulium has thirty-one radioactive isotopes. The most stable of these isotopes are thulium-171 and thulium-170, with half-lives of 1.92 years and 128.6 days, respectively. The remaining twenty-nine radioactive isotopes all have half-lives of just a few minutes or less. Unstable thulium isotopes with mass numbers less than 169 (thulium’s only stable isotope) undergo electron capture, a form of radioactive decay in which an atom’s electron is absorbed by the nucleus and combines with a proton to form a neutron and a neutrino. In thulium, this usually produces erbium isotopes. Unstable thulium isotopes with mass numbers greater than 169 undergo beta decay, another form of radioactive decay, in which a proton is converted into a neutron (or vice versa) inside the nucleus of an atom, releasing energy in the form of a beta particle (a fast-moving electron or positron). In thulium, this usually results in isotopes of ytterbium.

Applications

Despite the name "rare earth element," such elements are usually quite abundant on Earth, occurring naturally in many different sources. Thulium is the second least abundant rare earth metal, after promethium, and it exists in Earth’s crust at 0.5 parts per million. Thulium is not found freely in nature, but it is present in minerals such as monazite and gadolinite, alongside other rare earth elements. It is from monazite sand, which is made up of approximately 0.007 percent thulium, that the element is most often extracted through an ion exchange process. Before these more modern techniques were developed in the 1950s, rare earth metals were notoriously difficult (and therefore expensive) to isolate. Due to its low abundance, thulium is still quite expensive, and it is primarily mined in China, with about fifty metric tons produced annually. Thulium serves no major biological role, except as a potential metabolism stimulant. While it is generally considered to be nontoxic, metallic thulium powder is actually regarded as highly toxic if inhaled or ingested, and it can even cause spontaneous explosions. People who are exposed to radioactive isotopes of thulium can be at risk for radiation poisoning.

Thulium-170, one of the element’s radioactive isotopes, can be used in portable x-ray machines as well as in the treatment of prostate cancer. Thulium is also used in the creation of high-efficiency lasers that can be found primarily in the field of medicine and the military. Thulium ions are used in the production of euro notes; the ions glow blue under ultraviolet light, serving as an indication of whether or not a bill is counterfeit.

Bibliography

Atwood, David A., ed. The Rare Earth Elements: Fundamentals and Applications. Hoboken: Wiley, 2012. Print.

Emsley, John. Nature’s Building Blocks: An A–Z Guide to the Elements. 2nd ed. New York: Oxford UP, 2011. Print.

Haynes, William M., ed. CRC Handbook of Chemistry and Physics. 95th ed. Boca Raton: CRC, 2014. Print.

Krebs, Robert E. The History and Use of Our Earth’s Chemical Elements: A Reference Guide. 2nd ed. Westport: Greenwood, 2006. Print.

Lucas, Jacques, et al. Rare Earths: Science, Technology, Production and Use. Waltham: Elsevier, 2015. Print.

"Technical Data for Thulium." The Photographic Periodic Table of the Elements. Element Collection, n.d. Web. 12 Aug. 2015.