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History

The rarest of the naturally-occurring rare-earth metals, thulium was discovered by Swedish chemist Per Teodor Cleve in 1879 by looking for impurities in samples of erbia, an oxide of erbium (this was the same method Carl Gustaf Mosander earlier used to discover some other rare earth elements).  Cleve started by removing all of the known contaminants of erbia (Er₂O₃) and upon additional processing, obtained two new substances; one brown and one green.  The brown substance turned out to be the oxide of the element holmium and was named holmia.   The green substance was the oxide of an unknown element.   Cleve named the oxide thulia and its element thulium for the ancient name for Scandinavia, Thule.

Characteristics

Like others in the lanthanide series, thulium is silver in color but it is also very soft---soft enough to cut with a knife.  Thulium is the least abundant of the naturally occurring rare earth elements. 

Occurrence

The element is never found in nature in pure form, but it is found in small quantities in minerals with other rare earths.  It is principally extracted from monazite (~0.007% thulium) ores found in river sands through ion-exchange.  Newer ion-exchange and solvent extraction techniques have led to easier separation of the rare earths, which has yielded much lower costs for thulium production.  The metal can be isolated through reduction of its oxide with lanthanum metal or by calcium reduction in a closed container.   None of thulium's compounds are commercially important.

Applications

Thulium has been used to create lasers but high production costs have prevented other commercial uses from being developed. Other applications, real and potential, include:

• When stable thulium (Tm-169) is bombarded in a nuclear reactor it can later serve as a radiation source in portable x-ray devices.
• The unstable isotope Tm-171 could possibly be used as an energy source.
• Tm-169 has potential use in ceramic magnetic materials called ferrites, which are used in microwave equipment.

Metallic thulium is relatively expensive and has only recently become available.  It currently has no commercial applications.

Compounds

Thulium forms no commercially important compounds.  Some of thulium's compounds include: thulium oxide (Tm₂O₃), thulium fluoride (TmF₃) and thulium iodide (TmI₃).

Isotopes

Naturally occurring thulium is composed of 1 stable isotope,  ¹⁶⁹Tm (100% natural abundance.  34 radioisotopes have been characterized, with the most stable being ¹⁷¹Tm with a half-life of 1.92 years, ¹⁷⁰Tm with a half-life of 128.6 days, ¹⁶⁸Tm with a half-life of 93.1 days, and ¹⁶⁷Tm with a half-life of 9.25 days.  All of the remaining radioactive isotopes have half-lifes that are less than 64 hours, and the majority of these have half lifes that are less than 2 minutes.  This element also has 14 meta states, with the most stable being Tm-164m (t_½ 5.1 minutes), Tm-160m (t_½ 74.5 seconds) and Tm-155m (t_½ 45 seconds).

The isotopes of thulium range in atomic weight from 144.97007 amu (¹⁴⁵Tm) to 178.95534 amu (¹⁷⁹Tm).  The primary decay mode before the most abundant stable isotope, ¹⁶⁹Tm, is electron capture, and the primary mode after is beta emission.  The primary decay products before ¹⁶⁹Tm are element erbium-68 isotopes, and the primary products after are element  ytterbium-70 isotopes.

Precautions

Thulium has a low-to-moderate acute toxic rating and should be handled with care.   Metallic thulium in dust form presents a fire and explosion hazard.