Thulium – facts and properties

Position in the periodic table

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Occurrence and extraction

Thulium is one of the heavy rare earths, introduced on the lanthanum page. After the all but absent promethium it is the scarcest lanthanide in the Earth's crust, yet still more common than silver, mercury or gold. It sits in very small shares in xenotime, monazite and the ion-adsorption clays of southern China, mostly well under one per cent of the rare-earth content. Because there is so little of it and separating it from its neighbours is laborious, thulium counts among the more expensive lanthanides. The metal is made by reducing the oxide with calcium in a closed crucible.

The discovery again belongs to the Swede Per Teodor Cleve. In 1879 he split the erbium earth into three parts. Besides true erbium oxide he found a brown oxide, holmium, and a greenish one that he called thulia, after Thule, the legendary land of the far north. How pure the new element was remained open for a long time. Only at the start of the twentieth century did the American Charles James obtain a largely pure thulium salt, after more than ten thousand recrystallisations.

Properties and reactions

Thulium is the thirteenth lanthanide and third from last. In period 6, erbium is on its left and ytterbium on its right. The electron configuration of thulium is [Xe] 4f¹³ 6s² and its electronegativity is 1.25. The silvery grey metal is soft enough to cut with a knife and has a density of 9.32 g/cm³ and an atomic radius of 174 pm. It melts at 1545 °C. The boiling point of 1950 °C is strikingly low, well below that of erbium and lutetium. The metal tarnishes slowly in air. Almost all compounds contain triply charged thulium. Divalent compounds are rare and unstable.

Its isotopes are a peculiarity. Natural thulium consists of one kind only, the stable thulium-169. That makes it a single-isotope element, and it eases the targeted production of a radioactive isotope: if the metal is bombarded with neutrons in a nuclear reactor, some of the nuclei capture one and become thulium-170.

Tm³⁺ ions are pale green, and in phosphors they give off blue light once they have taken up ultraviolet light. Here too the transitions come from inside the 4f subshell.

Uses in everyday life and technology

Thulium is needed only in small amounts, but for special jobs. The artificial thulium-170 has a half-life of about four months. As it decays it emits beta radiation, and when that is slowed down in the surrounding metal it produces soft X-rays. This principle is built into portable X-ray units that need no mains supply and help, for example, with materials testing on site. The source is shielded and handled only by trained staff.

Better known today is the thulium fibre laser. Its glass fibre contains thulium and emits light around 2000 nanometres. The water in tissue absorbs this wavelength strongly, so the energy stays in the topmost layer. In surgery, especially urology, it cuts tissue and seals small blood vessels at the same time. In manufacturing this light welds transparent plastics, because they absorb exactly this radiation while the light of other lasers passes straight through.

Its glow has a use too. Calcium sulfate doped with thulium serves in dosimeters that measure how much radiation workers receive: it stores energy from the radiation and gives it back as light when heated. The amount of light reveals the dose.

Quick question

What is the atomic number of thulium (Tm)?

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