Terbium (Tb)

• Element Symbol: Tb
• Atomic Number: 65
• Atomic Mass: 158.9253
• Group # in Periodic Table: n/a
• Group Name: Lanthanides
• Period in Periodic Table: 6
• Block of Periodic Table: f-block
• Discovered by: Carl Gustaf Mosander (1843)

Terbium 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 yttrium, europium, and thulium. Rare earth elements typically occur together in nature and are oftentimes difficult to separate.

In 1787 a mineral called gadolinite was discovered in a quarry outside the small Swedish town of Ytterby. It was from this mineral that the first rare earth element, yttrium, was discovered in 1789 by Johan Gadolin, and the element was named for the town of its mineral source. As would later prove common with rare earth metals, it was believed that this sample of yttrium contained additional elements. In 1843 this was proved to be true when Swedish chemist Carl Gustaf Mosander successfully separated two new materials from yttrium. He called these new materials "erbia," the oxide of the element erbium, and "terbia," the oxide of the new element terbium. In total, there are four elements that are named for the town of Ytterby, Sweden: yttrium, ytterbium, erbium, and terbium. A pure sample of terbium was not isolated until the modern technique of solvent extraction was developed in the 1950s.

Physical Properties

Terbium is a silvery-gray metal. At 298 kelvins (K), its standard state is a ductile solid, with a density of 8.23 grams per cubic centimeter (g/cm³). In this state terbium is soft and malleable, and it can be scraped with a knife. Terbium is very susceptible to corrosion when exposed to air and quickly oxides at room temperature. The melting point of terbium is 1356 degrees Celsius (°C). Its boiling point is 3230 °C. The specific heat of terbium, at 298 K, is 182 joules per kilogram-kelvin (J/kg·K). Terbium is a good conductor, with an electrical conductivity of 8.3 × 10⁵ siemens per meter (S/m). It has a thermal conductivity of 11 watts per meter-kelvin (W/m·K). At temperatures at or below −54.15 °C, terbium is considered ferromagnetic (the strongest type of magnetism); between −54.15 °C and −43.15 °C, it becomes weaker and antiferromagnetic; and at temperatures above −43.15 °C, it becomes paramagnetic (only magnetized when placed within a magnetic field).

Chemical Properties

The most common oxidation states of terbium are +4 and +3. Terbium reacts slowly with cold water but vigorously when in contact with hot water. Terbium oxide takes on the form of a white-colored powder, while other compounds are dark maroon. When terbium ions are excited, they emit a bright green color. Terbium has a hexagonal close-packed crystal structure, but the element undergoes a crystal transformation at a temperature of 1289 °C.

Naturally occurring terbium consists solely of terbium-159, the element’s only stable isotope. Thirty-six other radioactive isotopes exist, the most stable of which are terbium-158 and terbium-157, with half-lives of 180 years and 71 years, respectively. The remaining isotopes all have half-lives that are less than ninety days, and in most cases less than thirty seconds. Unstable terbium isotopes with mass numbers less than 159 (terbium’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 terbium, this usually produces gadolinium isotopes. Unstable terbium isotopes with mass numbers greater than 159 undergo beta-minus decay, a specific form of beta decay that results in the production of an electron and an electron antineutrino; in terbium, this process usually results in isotopes of dysprosium.

Applications

Despite the name "rare earth element," such elements are usually quite abundant on Earth, occurring naturally in many different sources. Terbium is not found freely in nature, but it is present in many rare earth minerals, including cerite, gadolinite, and monazite. Terbium is commercially extracted via ion exchange from monazite, which is approximately 0.03 percent terbium, and from euxenite, which contains more than 1 percent terbium. Before these modern techniques were adopted in the 1950s, rare earth metals were notoriously difficult (and therefore expensive) to isolate. China is the world’s largest producer of the element, and because terbium is still quite expensive to obtain, only about ten metric tons of it are mined annually. Terbium is found in Earth’s crust at 1.1 parts per million, making it twice as abundant as silver. It serves no biological role, but it can be an irritant to the skin and eyes. Not much is known about the toxicity of terbium, and although it is considered to have low toxicity, it must still be handled with extreme care.

Terbium’s ability to emit a green luminescence upon excitation plays a role in the majority of its commercial applications. For this reason, terbium is used to make phosphors that create the green color that exists in television tubes as well as in fluorescent and trichromatic lighting. Additionally, terbium is used in the production of euro notes, along with thulium and europium; when placed under ultraviolet light, the terbium ions present in the bills glow green, aiding in the detection of counterfeit bills (thulium glows blue and europium red). Terbium is also utilized in alloys with dysprosium and iron, which lengthens and shortens within a magnetic field, making it useful in the production of certain electronic devices, such as loudspeakers and sonar systems.

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 Terbium." The Photographic Periodic Table of the Elements. Element Collection, n.d. Web. 13 Aug. 2015.