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Thulium is a chemical element; it has symbol Tm and atomic number 69. It is the thirteenth element in the lanthanide series of . It is the second-least abundant lanthanide in the Earth's crust, after radioactively unstable promethium. It is an easily workable metal with a bright silvery-gray luster. It is fairly soft and slowly in air. Despite its high price and rarity, thulium is used as a dopant in solid-state lasers, and as the radiation source in some portable X-ray devices. It has no significant biological role and is not particularly toxic.
In 1879, the Swedish chemist Per Teodor Cleve separated two previously unknown components, which he called holmia and thulia, from the rare-earth mineral erbia; these were the oxides of holmium and thulium, respectively. His example of thulium oxide contained impurities of ytterbium oxide. A relatively pure sample of thulium oxide was first obtained in 1911. The metal itself was first obtained in 1936 by Wilhelm Klemm and Heinrich Bommer..
Like the other lanthanides, its most common oxidation state is +3, seen in its oxide, halides and other compounds. In aqueous solution, like compounds of other late lanthanides, soluble thulium compounds form coordination complexes with nine water molecules.
Thulium has two major : the tetragonal α-Tm and the more stable hexagonal β-Tm.
Thulium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form thulium hydroxide:
Thulium reacts with all the . Reactions are slow at room temperature, but are vigorous above 200°C:
Thulium dissolves readily in dilute sulfuric acid to form solutions containing the pale green Tm(III) ions, which exist as complexes:
Thulium reacts with various metallic and non-metallic elements forming a range of binary compounds, including , , , , , , , , , and . Like most lanthanides, the +3 state is most common and is the only state observed in thulium solutions. Thulium exists as a ion in solution. In this state, the thulium ion is surrounded by nine molecules of water. ions exhibit a bright blue luminescence. Because it occurs late in the series, the +2 oxidation state can also exist, stabilized by the nearly full 4f electron shell, but occurs only in solids.
Thulium's only known oxide is thulium oxide. This oxide is sometimes called "thulia". Reddish-purple thulium(II) compounds can be made by the reduction of thulium(III) compounds. Examples of thulium(II) compounds include the halides (except the fluoride). Some hydrated thulium compounds, such as and are green or greenish-white. Thulium dichloride reacts very vigorously with water. This reaction results in hydrogen gas and exhibiting a fading reddish color. Combination of thulium and chalcogens results in thulium .
Thulium reacts with hydrogen chloride to produce hydrogen gas and thulium chloride. With nitric acid it yields thulium nitrate, or .
Thulium-169 is thulium's only primordial isotope and is the only isotope of thulium that is thought to be stable; it is predicted to undergo alpha decay to holmium-165 with a very long half-life. The longest-lived radioisotopes are thulium-171, which has a half-life of 1.92 years, and thulium-170, which has a half-life of 128.6 days. Most other isotopes have half-lives of a few minutes or less.
In total, 40 isotopes and 26 of thulium have been detected. Most isotopes of thulium lighter than decay via electron capture or beta-plus decay, although some exhibit significant alpha decay or proton emission. Heavier isotopes undergo beta-minus decay.
Thulium was so rare that none of the early workers had enough of it to purify sufficiently to actually see the green color; they had to be content with spectroscopy observing the strengthening of the two characteristic absorption bands, as erbium was progressively removed. The first researcher to obtain nearly pure thulium was Charles James, a British expatriate working on a large scale at New Hampshire College in Durham, USA. In 1911 he reported his results, having used his discovered method of bromate fractional crystallization to do the purification. He famously needed 15,000 purification operations to establish that the material was homogeneous.
High-purity thulium oxide was first offered commercially in the late 1950s, as a result of the adoption of ion exchange separation technology. Lindsay Chemical Division of American Potash & Chemical Corporation offered it in grades of 99% and 99.9% purity. The price per kilogram oscillated between US$4,600 and $13,300 in the period from 1959 to 1998 for 99.9% purity, and it was the second highest for the lanthanides behind lutetium.
Thulium makes up approximately 0.5 parts per million of soil, although this value can range from 0.4 to 0.8 parts per million. Thulium makes up 250 parts per quadrillion of seawater. In the Solar System, thulium exists in concentrations of 200 parts per trillion by weight and 1 part per trillion by moles. Thulium ore occurs most commonly in China. Australia, Brazil, Greenland, India, Tanzania, and the United States also have large reserves of thulium. In 2001, the total world reserves of thulium were approximately 100,000 . Thulium is the least abundant lanthanide on Earth except for the radioactive promethium.
The metal can be isolated through redox of its oxide with lanthanum metal or by calcium reduction in a closed container. None of thulium's natural compounds are commercially important. In 2001, approximately 50 tonnes per year of thulium oxide were produced. In 1996, thulium oxide cost US$20 per gram, and in 2005, 99%-pure thulium metal powder cost US$70 per gram.
The blue fluorescence of Tm-doped calcium sulfate has been used in personal dosimeters for visual monitoring of radiation. Tm-doped halides in which Tm is in its 2+ oxidation state are luminescent materials that are proposed for electric power generating windows based on the principle of a luminescent solar concentrator.
In humans, thulium occurs in the highest amounts in the liver, , and . Humans typically consume several micrograms of thulium per year. The roots of do not take up thulium, and the dry matter of vegetables usually contains one part per billion of thulium. Thulium metal has low to moderate toxicity. Thulium dust can cause and fires.
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