Thulium is a chemical element; it has symbol Tm and atomic number 69. It is the thirteenth element in the lanthanide series of metals. 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 tarnishes in air. Despite its high price and rarity, thulium is used as a dopant in solid-state lasers. It has no significant biological role and is not particularly toxic. Artificial radioactive isotopes of thulium are used as radiation sources in some portable X-ray devices. 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.
Properties
Physical properties Pure thulium metal has a bright, silvery luster, which tarnishes on exposure to air. The metal can be cut with a knife, as it has a Mohs hardness of 2 to 3; it is malleable and ductile. Thulium is ferromagnetic below 32 K, antiferromagnetic between 32 and 56 K, and paramagnetic above 56 K. Thulium has two major allotropes: the tetragonal α-Tm and the more stable hexagonal β-Tm.
Chemical properties Thulium tarnishes slowly in air and burns readily at 150 °C to form thulium(III) oxide:
4 Tm + 3 O2 → 2 Tm2O3 Thulium dust can cause explosions and fires. Thulium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form thulium hydroxide:
2 Tm(s) + 6 H2O(l) → 2 Tm(OH)3(aq) + 3 H2(g) Thulium reacts with all the halogens. Reactions are slow at room temperature, but are vigorous above 200 °C:
2 Tm(s) + 3 F2(g) → 2 TmF3(s) (white) 2 Tm(s) + 3 Cl2(g) → 2 TmCl3(s) (yellow) 2 Tm(s) + 3 Br2(g) → 2 TmBr3(s) (white) 2 Tm(s) + 3 I2(g) → 2 TmI3(s) (yellow) Thulium dissolves readily in dilute sulfuric acid to form solutions containing the pale green Tm(III) ions, which exist as [Tm(OH2)9]3+ complexes:
2 Tm(s) + 3 H2SO4(aq) → 2 Tm3+(aq) + 3 SO42−(aq) + 3 H2(aq) Thulium reacts with various metallic and non-metallic elements forming a range of binary compounds, including TmN, TmS, TmC2, Tm2C3, TmH2, TmH3, TmSi2, TmGe3, TmB4, TmB6 and TmB12. Like most lanthanides, the +3 state is most common and is the only state observed in thulium solutions. Thulium exists as a Tm3+ ion in solution. In this state, the thulium ion is surrounded by nine molecules of water. Tm3+ 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 Tm2O3. 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 TmCl3·7H2O and Tm2(C2O4)3·6H2O are green or greenish-white. Thulium dichloride reacts very vigorously with water. This reaction results in hydrogen gas and Tm(OH)3 exhibiting a fading reddish color. Combination of thulium and chalcogens results in thulium chalcogenides. Thulium reacts with hydrogen chloride to produce hydrogen gas and thulium chloride. With nitric acid it yields thulium nitrate, Tm(NO3)3.
Isotopes
Natural thulium consists of the single observationally stable isotope thulium-169, which is predicted to undergo alpha decay to holmium-165 with a very long half-life. Known isotopes of thulium range from 144Tm to 183Tm. The primary decay mode before the stable isotope, 169Tm, is electron capture to erbium isotopes, and the primary mode after is beta emission to ytterbium isotopes. 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 under 10 minutes.
History
Thulium was discovered by Swedish chemist Per Teodor Cleve in 1879 by looking for impurities in the oxides of other rare earth elements. 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). Upon additional processing, he obtained two new substances; one brown and one green. The brown substance was the oxide of the element holmium and was named holmia by Cleve, and the green substance was the oxide of an unknown element. Cleve named the oxide thulia and its element thulium after Thule, an Ancient Greek place name associated with Scandinavia or Iceland. Thulium's atomic symbol was initially Tu, but later changed to Tm to avoid confusion with tungsten which was commonly written as Tu around the time of thulium's discovery. 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 spectroscopically 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.
Occurrence
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