1s² 2s²p⁶ 3s²p⁶d¹⁰ 4s²p⁶d¹⁰ 5s²p⁶d¹ 6s²

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, lanthanein, "to lie hidden".  That is an apt description since lanthanum generally occurs along with other so-called rare earth elements and is very difficult to separate. The abundance of the metal is similar to that of zinc or nickel.

Characteristics

Lanthanum is a silvery white metallic element belonging to group 3 of the periodic table and often considered to be one of the lanthanides.  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.

For that reason the metal is usually stored under oil or kerosene. Most lanthanum is extracted from monazite sands.  Lanthanum is used in the electrodes for high-intensity carbon-arc lamps and also in the production of high-purity europium metal (element 63). 

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).

Monazite (Ce, La, Th, Nd, Y)PO₄, and bastnasite (Ce, La, Y)CO₃F, are principal ores in which lanthanum occurs in percentages up to 25 to 38 percent.

Applications

• Carbon lighting applications, especially by the motion picture industry for studio lighting and projection.
• La₂O₃ improves the alkali resistance of glass, and is used in making special optical glasses, such as:
  • Infrared absorbing glass.
  • Camera and telescope lenses, because of the high refractive index and low dispersion of rare-earth glasses.
• Small amounts of lanthanum added to steel improves its malleability, resistance to impact and ductility. 
• Small amounts of lanthanum added to iron helps to produce nodular cast iron.
• Small amounts of lanthanum added to molybdenum decreases the hardness of this metal and its sensitivity to temperature variations.
• Mischmetal, a pyrophoric alloy used e.g. in lighter flints, contains 25% to 45% lanthanum.
• Lanthanum oxide and the boride are used in electronic vacuum tubes as hot cathode materials with strong emissivity of electrons.  Crystals of LaB₆ are used in high brightness, extended life, thermionic electron emission sources for scanning electron microscopes.
• in Gas tungsten arc welding electrodes, as a substitute for radioactive thorium. 
• Hydrogen spong alloys can contain lanthanum. These alloys are capable of storing up to 400 times their own volume of hydrogen gas in a reversible adsorption process.
• Petroleum cracking catalysts.
• Gas lantern mantles. 
• Glass and lapidary polishing compound.
• La-Ba age dating of rocks and ores.
• Lanthanum carbonate is used medically as a phosphate binder for the treatment of hyperphosphatemia.
• Lanthanum nitrate is mainly applied in specialty glass, water treatment and catalyst.
• Cerium activated Lanthanum bromide is the recent inorganic scintillator which has a combination of high light yield and the best energy resolution.
• Like horseradish peroxidase, lanthanum is used as an electron-dense tracer in molecular biology.

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.

Compounds

Isotope

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. 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 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.

(Gr. lanthanein, to lie hidden) Mosander in 1839 extracted a new earth lanthana, from impure cerium nitrate and recognized the new element. Lanthanum is found in rare-earth minerals such as cerite, monazite, allanite, and bastnasite. Monazite and bastnasite are principal ores in which lanthanum occurs in percentages up to 25 and 38% respectively. Misch metal, used in making lighter flints, contains about 25% lanthanum. Lanthanum was isolated in relatively pure form in 1923, iron exchange and solvent extraction techniques have led to much easier isolation of the so-called "rare-earth" elements. The availability of lanthanum and other rare earths has improved greatly in recent years. the metal can be produced by reducing the anhydrous fluoride with calcium. Lanthanum is silvery white, malleable, ductile, and soft enough to be cut with a knife. It is one of the most reactive of the rare-earth metals. It oxidizes rapidly when exposed to air. Cold water attacks lanthanum slowly, and hot water attacks it much more rapidly. the metal reacts directly with elemental carbon, nitrogen, boron, selenium, silicon, phosphorus, sulfur, and with halogens. At 310^oC, lanthanum changes from a hexagonal to a face-centered cubic structure, and at 865^oC it again transforms into a body-centered cubic structure. Natural lanthanum is a mixture of two stable isotopes, 138La and 139La. Twenty three other radioactive isotopes are recognized. Rare-earth compounds containing lanthanum are extensively used in carbon lighting applications, especially by the motion picture industry for studio lighting and projection. This application consumes about 25% of the rare-earth compounds produced. La₂O₃ improves the alkali resistance of glass, and is used in making special optical glasses. Small amounts of lanthanum, as an additive, can be used to produce nodular cast iron. There is current interest in hydrogen sponge alloys containing lanthanum. These alloys take up to 400 times their own volume of hydrogen gas, and the process is reversible. Heat energy is released every time that they do so; therefore these alloys have possibilities in energy conservation system. Lanthanum and its compounds have a low to moderate acute toxicity rating; therefore, care should be taken in handling them. The metal costs about $5/g.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond