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Lutetium ( ) is a chemical element with the symbol Lu and atomic number 71. It is in the d-block of the periodic table, not the f-block, but the IUPAC classifies it as a lanthanoid. It is one of the elements that traditionally were included in the classification, "rare earths". One of its radioactive isotopes (176Lu) is used in nuclear technology to determine the age of meteorites. Lutetium usually occurs in association with the element yttrium and is sometimes used in metal alloys and as a catalyst in various chemical reactions.

Characteristics

Physical

Lutetium is a silvery white corrosion-resistant trivalent metal. It has the smallest atomic radius and is the heaviest and hardest of the rare earth elements.Lutetium has the highest melting point of any lanthanide, probably related to the lanthanide contraction.

Chemical

Lutetium metal tarnishes slowly in air and burns readily at 150 °C to form lutetium oxide:
4 Lu + 3 O2 → 2 Lu2O3


Lutetium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form lutetium hydroxide:
2 Lu (s) + 6 H2O (l) → 2 Lu(OH)3 (aq) + 3 H2 (g)


Lutetium metal reacts with all the halogens to form halides:
2 Lu (s) + 3 F2 (g) → 2 LuF3 (s)
2 Lu (s) + 3 Cl2 (g) → 2 LuCl3 (s)
2 Lu (s) + 3 Br2 (g) → 2 LuBr3 (s)
2 Lu (s) + 3 I2 (g) → 2 LuI3 (s)


The fluoride, chloride, and bromide are white, whereas the iodide is brown.

Lutetium dissolves readily in dilute sulfuric acid to form solutions containing the colorless lutetium(III) ions, which exist as a [Lu(OH2)9]3+ complex:

2 Lu (s) + 3 H2SO4 (aq) → 2 Lu3+ (aq) + 3 SO (aq) + 3 H2 (g)


Compounds

In all its compounds, lutetium occurs in +3 valence state. Aqueous solutions of most Lu salts are colorless and form white crystalline solids upon drying. The soluble salts, such as chloride (LuCl3), bromide (LuBr3), iodide (LuI3), nitrate, sulfate and acetate form hydrates upon crystallization. The oxide (Lu2O3), hydroxide, fluoride (LuF3), carbonate, phosphate and oxalate are insoluble in water.

Lutetium tantalate (LuTaO4) is the densest known stable white material (density 9.81 g/cm3) and therefore is an ideal host for X-ray phosphors. Thoria is more dense (10 g/cm3) and is also white, but radioactive.

Isotopes

Naturally occurring lutetium is composed of 1 stable isotope 175Lu (97.41% natural abundance) and 1 long-lived beta-radioactive isotope 176Lu with a half-life of 3.78×1010 years (2.59% natural abundance). The last one is used in radiometric dating (see Lutetium-hafnium dating). 33 radioisotopes have been characterized, with the most stable being naturally occurring 176Lu, and artificial isotopes 174Lu with a half-life of 3.31 years, and 173Lu with a half-life of 1.37 years. All of the remaining radioactive isotopes have half-lives that are less than 9 days, and the majority of these have half-lives that are less than half an hour. This element also has 18 meta states, with the most stable being 177mLu (T½=160.4 days), 174mLu (T½=142 days) and 178mLu (T½=23.1 minutes).

The known isotopes of lutetium range in atomic weight from 149.973 (150Lu) to 183.961 (184Lu). The primary decay mode before the most abundant stable isotope, 175Lu, is electron capture (with some alpha and positron emission), and the primary mode after is beta emission. The primary decay products before 175Lu are element 70 (ytterbium) isotopes and the primary products after are element 72 (hafnium) isotopes.

History

Lutetium (Latin Lutetiamarker meaning Parismarker) was independently discovered in 1907 by French scientist Georges Urbain, Austrian mineralogist Baron Carl Auer von Welsbach, and American chemist Charles James. All of these men found lutetium as an impurity in the mineral ytterbia which was thought by Swiss chemist Jean Charles Galissard de Marignac (and most others) to consist entirely of the element ytterbium.

The separation of lutetium from Marignac's ytterbium was first described by Urbain and the naming honor therefore went to him. He chose the names neoytterbium (new ytterbium) and lutecium for the new element but neoytterbium was eventually reverted back to ytterbium and in 1949 the spelling of element 71 was changed to lutetium.

The dispute on the priority of the discovery is documented in two articles in which Urbain and von Welsbach accuse each other of publishing results influenced by the published research of the other.

The Commission on Atomic Mass, which was responsible for the attribution of the names for the new elements, settled the dispute in 1909 by granting priority to Urbain and adopting his names as official ones. An obvious problem with this decision was that Urbain was one of the four members of the commission.

Welsbach proposed the names cassiopium for element 71 (after the constellation Cassiopeia) and aldebaranium for the new name of ytterbium but these naming proposals were rejected (although many German scientists in the 1950s called the element 71 cassiopium). The irony of all this is that Charles James, who had modestly stayed out of the argument as to priority, worked on a much larger scale than the others, and undoubtedly possessed the largest supply of lutetium at the time.

Occurrence and production

Monazite
Found with almost all other rare-earth metals but never by itself, lutetium is very difficult to separate from other elements. The principal commercially viable ore of lutetium is the rare earth phosphate mineral monazite: (Ce, La, etc.) PO4 which contains 0.003% of the element. The abundance of lutetium in the Earth crust is only about 0.5 mg/kg. The main mining areas are Chinamarker, United Statesmarker, Brazilmarker, Indiamarker, Sri Lankamarker and Australia. The world production of lutetium (in the form of oxide) is about 10 tonnes per year. Pure lutetium metal has only relatively recently been isolated and is very difficult to prepare. It is one of the rarest and most expensive of the rare earth metals with the price about US$ 10,000 per kg.

Crushed minerals are treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. Thorium precipitates out of solution as hydroxide and is removed. After that the solution is treated with ammonium oxalate to convert rare earths in to their insoluble oxalates. The oxalates are converted to oxides by annealing. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in HNO3. Several rare earth metals, including Lu, are separated as a double salt with ammonium nitrate by crystallization. Lutetium is separated by ion exchange. In this process, rare-earth ions are sorbed onto suitable ion-exchange resin by exchange with hydrogen, ammonium or cupric ions present in the resin. Lutetium salts are then selectively washed out by suitable complexing agent. Lutetium metal is then obtained by reduction of anhydrous LuCl3 or LuF3 by either an alkali metal or alkaline earth metal.

2 LuCl3 → 2 Lu + 3 CaCl2


Applications

Because of the rarity and high price, lutetium has very few commercial uses. However, stable lutetium can be used as catalysts in petroleum cracking in refineries and can also be used in alkylation, hydrogenation, and polymerization applications.

Some other applications include:

Precautions

Like other rare-earth metals, lutetium is regarded as having a low degree of toxicity, but its compounds should be handled with care nonetheless. Metal dust of this element is a fire and explosion hazard. Lutetium plays no biological role in the human body.

References



  • Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1


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