Definitions

lanthanum

lanthanum

[lan-thuh-nuhm]
lanthanum [Gr.,=to lie hidden], metallic chemical element; symbol La; at. no. 57; at. wt. 138.9055; m.p. about 920°C;; b.p. about 3,460°C;; sp. gr. 6.19 at 25°C;; valence +3. Lanthanum is a soft, malleable, ductile, silver-white metal; at room temperature it has a hexagonal close-packed crystalline structure that is unstable at higher temperatures (see allotropy). Lanthanum is usually considered the first member of the lanthanide series, a group of elements with similar physical and chemical properties. It is one of the rare-earth metals of Group 3 of the periodic table. Lanthanum is a chemically active element. It oxidizes rapidly in air and reacts with water to form the hydroxide. It reacts readily with acids, with elemental boron, carbon, nitrogen, phosphorus, selenium, silicon, or sulfur, and with the halogens. The oxide and the boride are used in electronic vacuum tubes. The oxide is added to optical glass to increase its alkali resistance and refractive index. Although lanthanum is not found uncombined in nature, it occurs in the rare-earth minerals monazite and bastnasite. Lanthanum may be prepared by reduction of lanthanum fluoride with calcium metal. Lanthanum may be used in making ductile cast iron; alloyed with other metals, it is used in cigarette lighter flints. Natural lanthanum is a mixture of two stable isotopes. One radioactive byproduct of the fission of plutonium, thorium, or uranium is a mixture of radioactive isotopes of lanthanum; 17 radioactive isotopes are known. Lanthanum was discovered in the form lanthanium oxide, called lanthana, in 1839 by C. G. Mosander.
Lanthanum is a chemical element with the symbol La and atomic number 57.

Characteristics

Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is a lanthanoid. Found in some rare-earth minerals, usually in combination with cerium and other rare earth elements. Lanthanum is malleable, ductile, and soft enough to be cut with a knife. It is one of the most reactive of the rare-earth metals. The metal reacts directly with elemental carbon, nitrogen, boron, selenium, silicon, phosphorus, sulfur, and with halogens. It oxidizes rapidly when exposed to air. Cold water attacks lanthanum slowly, while hot water attacks it much more rapidly.

Applications

Uses of lanthanum include:

History

Lanthanum was discovered in 1839 by Swedish chemist Carl Gustav Mosander, when he partially decomposed a sample of cerium nitrate by heating and treating the resulting salt with dilute nitric acid. From the resulting solution, he isolated a new rare earth he called lantana. Lanthanum was isolated in relatively pure form in 1923.

The word lanthanum comes from the Greek λανθανω [lanthanō] = to lie hidden.

Lanthanum is the most strongly basic of all the trivalent lanthanoids, and this property is what allowed Mosander to isolate and purify the salts of this element. Basicity separation as operated commercially involved the fractional precipitation of the weaker bases (such as didymium) from nitrate solution by the addition of magnesium oxide or dilute ammonia gas. Purified lanthanum remained in solution. (The basicity methods were only suitable for lanthanum purification; didymium could not be efficiently further separated in this manner.) The alternative technique of fractional crystallization was invented by Dimitry Mendeleev himself, in the form of the double ammonium nitrate tetrahydrate, which he used to separate the less-soluble lanthanum from the more-soluble didymium in the 1870s. This system would be used commercially in lanthanum purification until the development of practical solvent extraction methods that started in the late 1950s. (A detailed process using the double ammonium nitrates to provide 4N pure lanthanum, neodymium concentrates and praseodymium concentrates is presented in Callow 1967, at a time when the process was just becoming obsolete.) As operated for lanthanum purification, the double ammonium nitrates were recrystallized from water. When later adapted by Carl Auer von Welsbach for the splitting of didymium, nitric acid was used as solvent to lower the solubility of the system. Lanthanum is relatively easy to purify, since it has only one adjacent lanthanoid, cerium, which itself is very readily removed due to its potential tetravalency.

Biological role

Lanthanum has no known biological role. The element is not absorbed orally, and when injected its elimination is very slow. Lanthanum carbonate was approved as a medication (Fosrenol, Shire Pharmaceuticals) to absorb excess phosphate in cases of end-stage renal failure. Some rare-earth chlorides, such as lanthanum chloride (LaCl3), are known to have anticoagulant properties.

While Lanthanum has pharmacological effects on several receptors and ion channels, its specificity for the GABA receptor is unique among divalent cations. Lanthanum acts at the same modulatory site on the GABAR as zinc- a known negative allosteric modulator. The Lanthanum cation La3+ is a positive allosteric modulator at native and recombinant GABA receptors, increasing open channel time and decreasing desensitization in a subunit configuration dependent manner.

Occurrence

Although lanthanum belongs to chemical elements group called rare earth metals, it is not rare at all. Lanthanum is available in relatively large quantities (32 ppm in Earth’s crust). "Rare earths" got their name since they were indeed rare as compared to the "common" earths such as lime or magnesia, and historically only a few deposits were known.

Monazite (Ce, La, Th, Nd, Y)PO4, and bastnäsite (Ce, La, Y)CO3F, are the principal ores in which lanthanum occurs, in percentages of up to 25 to 38 percent of the total lanthanoid content. Lanthanum is more generally enriched in bastnäsite as opposed to monazite, in commercial orebodies. Until 1949, bastnäsite was a rare and obscure mineral, not even remotely contemplated as a potential commercial source for lanthanoids. In that year, the vast deposit at Mountain Pass, California was discovered. This discovery alerted geologists as to the existence of a whole new class of rare earth deposit, the rare-earth bearing carbonatite, other examples of which soon surfaced, particularly in Africa and China.

See also Lanthanide minerals

Isotopes

Naturally occurring lanthanum is composed of one stable (139La) and one radioactive (138La) isotope, with the stable isotope, 139La, being the most abundant (99.91% natural abundance). 38 radioisotopes have been characterized with the most stable being 138La with a half-life of 105×109 years, and 137La with a half-life of 60,000 years. Most of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half lives that are less than 1 minute. This element also has 3 meta states.

The isotopes of lanthanum range in atomic weight from 117 u (117La) to 155 u (155La).

Precautions

Lanthanum has a low to moderate level of toxicity, and should be handled with care. In animals, the injection of lanthanum solutions produces glycaemia, low blood pressure, degeneration of the spleen and hepatic alterations.

References

  • Los Alamos National Laboratory – Lanthanum
  • "The Industrial Chemistry of the Lanthanons, Yttrium, Thorium and Uranium", by R.J. Callow, Pergamon Press 1967
  • "Chemistry of the Lanthanons", by R.C. Vickery, Butterworths 1953
  • "Nouveau Traite de Chimie Minerale, Vol. VII. Scandium, Yttrium, Elements des Terres Rares, Actinium", P. Pascal, Editor, Masson & Cie 1959
  • "Extractive Metallurgy of Rare Earths", by C.K. Gupta and N. Krishnamurthy, CRC Press 2005

See also

Lanthanum compounds

External links

Search another word or see lanthanumon Dictionary | Thesaurus |Spanish
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature
FAVORITES
RECENT

;