Thulium

Thulium is an element that took a lot of trial and error to discover. Its discovery can be traced back to 1789, when Swedish chemist Johan Gadolin discovered the element yttrium (in the form of yttrium oxide, or yttria) in an ore from near the village of Ytterby, Sweden. However, Gadolin’s yttria wasn’t pure yttrium.

In 1843, Swedish chemist Carol Gustaf Mosander discovered terbium and erbium in the oxide, too. But these oxides weren’t pure, either! In 1879, Swedish chemist Per Teodor Cleve discovered the elements holmium and thulium in a sample of erbium oxide. Cleve named element 69 “thulium” after the Latin word for Scandinavia, thule (which you might also recognize as a manufacturer of automobile roof racks).

Thulium was difficult to discover in part because it’s a rare-earth element. The rare-earth elements all have similar qualities, and they’re difficult to separate from each other. In fact, it wasn’t until 1911 that American chemist Theodore William Richards succeeded in obtaining a pure sample of thulium. He did this by painstakingly recrystallizing thulium bromate 15,000 times! In recognition of his efforts, Richards was awarded the Nobel Prize for chemistry in 1914.

Like other rare-earth elements, thulium is a metal soft enough to cut with a knife. It is malleable (meaning it can be shaped by hammering) and ductile (meaning it can be stretched without breaking). Thulium tarnishes slowly in air. The element interacts with water to make thulium hydroxide.  

Thulium isn’t one of those elements that has a million uses, partly because the element is so expensive to produce. Other rare-earth elements are far cheaper and similar enough to thulium that in most cases, they can be used instead of thulium.

Thulium has been added to yttrium aluminum garnet—an artificial crystal that’s in the kinds of lasers that doctors use in laser surgery. Thulium-170, an isotope that is produced in laboratories, can be used in portable X-ray devices.

Thulium forms just one oxide (Tm₂O₃), and this substance is often referred to as “thulia” (in keeping with the names of other rare-earth oxides, such as yttria, erbia, and terbia). While thulium does form compounds such as chlorides, fluorides, and bromides, none of these have major commercial uses. However, thulium iodide has been used to add a bluish green color to lamps and lights.

In nature, you’ll find thulium in just one stable (non-radioactive) isotope—thulium-169. Scientists have characterized some 25 radioactive thulium isotopes. The longest-lived of these, thulium-171, has a half-life of nearly two years. The shortest-lived have half-lives of mere nanoseconds.

Most thulium comes from China—specifically, from the rare earth mines in Inner Mongolia, which are the source of most of the world’s rare earth elements. Thulium is extracted chiefly from the minerals monazite and bastnäsite. Russia and Malaysia also produce thulium.

Not all the rare-earth elements are in fact rare, but thulium is! Thulium occurs in tiny amounts in rare-earth minerals, which is why it’s one of the most expensive elements in the world.