|Name, Symbol, Number||lanthanum, La, 57|
|Group, Period, Block||3, 6, f|
|Atomic mass||138.90547(7) g/mol|
|Electron configuration||[Xe] 5d1 6s2|
|Electrons per shell||2, 8, 18, 18, 9, 2|
|Density (near r.t.)||6.162 g/cm³|
|Liquid density at m.p.||5.94 g/cm³|
|Melting point||1193 K
(920 °C, 1688 °F)
|Boiling point||3737 K
(3464 °C, 6267 °F)
|Heat of fusion||6.20 kJ/mol|
|Heat of vaporization||402.1 kJ/mol|
|Heat capacity||(25 °C) 27.11 J/(mol·K)|
(strongly basic oxide)
|Electronegativity||1.10 (Pauling scale)|
|1st: 538.1 kJ/mol|
|2nd: 1067 kJ/mol|
|3rd: 1850.3 kJ/mol|
|Atomic radius||195 pm|
|Covalent radius||169 pm|
|Electrical resistivity||(r.t.) (α, poly) 615 nΩ·m|
|Thermal conductivity||(300 K) 13.4 W/(m·K)|
|Thermal expansion||(r.t.) (α, poly)
|Speed of sound (thin rod)||(20 °C) 2475 m/s|
|Speed of sound (thin rod)||(r.t.) (α form) 36.6 m/s|
|Shear modulus||(α form) 14.3 GPa|
|Bulk modulus||(α form) 27.9 GPa|
|Poisson ratio||(α form) 0.280|
|Vickers hardness||491 MPa|
|Brinell hardness||363 MPa|
|CAS registry number||7439-91-0|
Lanthanum (chemical symbol La, atomic number 57) is a soft, silvery white metallic element. Found in combination with other rare earth elements, it is one of the most reactive of the rare earth metals.
Lanthanum, its alloys, and its compounds have a wide range of applications. For instance, the element is used in motion-picture studio illumination that relies on carbon-arc lighting, and it is a substitute for thorium in gas tungsten-arc welding electrodes. It is an important component of mischmetal, a pyrophoric alloy used in lighter flints, and small amounts of this element may be added to steel to improve its malleability, or to molybdenum to decrease its hardness. Lanthanum oxide is useful for special optical glasses, such as camera and telescope lenses and infrared-absorbing glass. The oxide and boride are used in electronic vacuum tubes, and the carbonate has been approved for a medication that absorbs excess phosphate in end-stage cases of kidney failure.
Although lanthanum belongs to a group of chemical elements called the rare earth metals, it is not rare at all. It is available in relatively large quantities—32 parts per million (ppm) in the Earth’s crust.
The principal ores containing lanthanum are monazite ((Ce, La, Th, Nd, Y)PO4) and bastnasite ((Ce, La, Y)CO3F). The proportion of lanthanum in these ores can be as high as 25–38 percent. It usually occurs in combination with cerium and other rare earth elements.
The word lanthanum comes from the Greek λανθανω [lanthanō], which means "to lie hidden."
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 resultant 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.
Lanthanum belongs to period six and group three (former group 3B) of the periodic table. It is often considered the first of the lanthanides, which are a group of "inner transition metals." From the standpoint of their electronic structure, the lanthanides are characterized by the gradual filling of the 4f subshell. Lanthanum, however, has no electrons in its 4f subshell, and it best fits with the elements of group three, corresponding to a group of transition metals.
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.
Naturally occurring lanthanum consists of one stable isotope (139La) and one radioactive (138La) isotope, with the stable isotope, 139La, being the most abundant (99.91 percent natural abundance). Many 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. All 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 one minute. The isotopes of lanthanum range in atomic weight from 117 u (117La) to 155 u (155La).
Lanthanum has no known biological role. The element is not absorbed orally, and when injected into experimental animals, its elimination is very slow. It has a low-to-moderate level of toxicity, and should be handled with care. In animals, the injection of lanthanum solutions produces glycemia, low blood pressure, degeneration of the spleen and hepatic alterations.
All links retrieved July 24, 2014.
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