Lanthanide
Lanthanum is a chemical element; it has symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the first and the prototype of the lanthanide series, a group of 15 similar elements between lanthanum and lutetium in the periodic table. Lanthanum is traditionally counted among the rare-earth elements. Like most other rare-earth elements, its usual oxidation state is +3, although some compounds are known with an oxidation state of +2. Lanthanum has no biological role in humans but is used by some bacteria. It is not particularly toxic to humans but does show some antimicrobial activity.
Lanthanum usually occurs together with cerium and the other rare-earth elements. Lanthanum was first found by the Swedish chemist Carl Gustaf Mosander in 1839 as an impurity in cerium nitrate – hence the name lanthanum, from the ancient Greek λανθάνειν (lanthanein), meaning 'to lie hidden'. Although it is classified as a rare-earth element, lanthanum is the 28th most abundant element in the Earth's crust, almost three times as abundant as lead. In minerals such as monazite and bastnäsite, lanthanum composes about a quarter of the lanthanide content. It is extracted from those minerals by a process of such complexity that pure lanthanum metal was not isolated until 1923. …
In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a heavy mineral from the mine at Bastnäs, later named cerite. Thirty years later, the fifteen-year-old Wilhelm Hisinger, from the family owning the mine, sent a sample of it to Carl Scheele, who did not find any new elements within. In 1803, after Hisinger had become an ironmaster, he returned to the mineral with Jöns Jacob Berzelius and isolated a new oxide which they named ceria after the dwarf planet Ceres, which had been discovered two years earlier. Ceria was simultaneously independently isolated in Germany by Martin Heinrich Klaproth. Between 1839 and 1843, ceria was shown to be a mixture of oxides by the Swedish surgeon and chemist Carl Gustaf Mosander, who lived in the same house as Berzelius and studied under him: he separated out two other oxides which he named lanthana and didymia. He partially decomposed a sample of cerium nitrate by roasting it in air and then treating the resulting oxide with dilute nitric acid. That same year, Axel Erdmann, a student also at the Karolinska Institute, discovered lanthanum in a new mineral from Låven island located in a Norwegian fjord.
Finally, Mosander explained his delay, saying that he had extracted a second element from cerium, and this he called didymium. Although he did not realise it, didymium too was a mixture, and in 1885 it was separated into praseodymium and neodymium. …
Lanthanum is the first element and prototype of the lanthanide series. In the periodic table, it appears to the right of the alkaline earth metal barium and to the left of the lanthanide cerium. Lanthanum is generally considered the first of the f-block elements by authors writing on the subject. The 57 electrons of a lanthanum atom are arranged in the configuration [Xe]5d16s2, with three valence electrons outside the noble gas core. In chemical reactions, lanthanum almost always gives up these three valence electrons from the 5d and 6s subshells to form the +3 oxidation state, achieving the stable configuration of the preceding noble gas xenon. Some lanthanum(II) compounds are also known, but they are usually much less stable. Lanthanum monoxide (LaO) produces strong absorption bands in some stellar spectra.
Among the lanthanides, lanthanum is exceptional as it has no 4f electrons as a single gas-phase atom. Thus it is only very weakly paramagnetic, unlike the strongly paramagnetic later lanthanides (with the exceptions of the last two, ytterbium and lutetium, where the 4f shell is completely full). However, the 4f shell of lanthanum can become partially occupied in chemical environments and participate in chemical bonding. …
Lanthanum makes up 39 mg/kg of the Earth's crust, behind neodymium at 41.5 mg/kg and cerium at 66.5 mg/kg. Despite being among the so-called "rare-earth metals", lanthanum is thus not rare at all, but it is historically named so because it is rarer than "common earths" such as lime and magnesia, and at the time it was recognized only a few deposits were known. Lanthanum is also considered a "rare-earth" metal because the process to mine it is difficult, time-consuming, and expensive. Lanthanum is rarely the dominant lanthanide found in the rare-earth minerals, and in their chemical formulas it is usually preceded by cerium. Rare examples of La-dominant minerals are monazite-(La) and lanthanite-(La).
The La3+ ion is similarly sized to the early lanthanides of the cerium group (those up to samarium and europium) that immediately follow in the periodic table, and hence it tends to occur along with them in phosphate, silicate and carbonate minerals, such as monazite (MIIIPO4) and bastnäsite (MIIICO3F), where M refers to all the rare-earth metals except scandium and the radioactive promethium (mostly Ce, La, and Y). Bastnäsite is usually lacking in thorium and the heavy lanthanides, and the purification of the light lanthanides from it is less involved. …
The first historical application of lanthanum was in gas lantern mantles. Carl Auer von Welsbach used a mixture of lanthanum oxide and zirconium oxide, which he called Actinophor and patented in 1886. The original mantles gave a green-tinted light and were not very successful, and his first company, which established a factory in Atzgersdorf in 1887, failed in 1889.
Modern uses of lanthanum include:
One material used for anodic material of nickel–metal hydride batteries is La(Ni3.6Mn0.4Al0.3Co0.7). Due to high cost to extract the other lanthanides, a mischmetal with more than 50% of lanthanum is used instead of pure lanthanum. The compound is an intermetallic component of the AB5 type. NiMH batteries can be found in many models of the Toyota Prius sold in the US. These larger nickel-metal hydride batteries require massive quantities of lanthanum for the production. The 2008 Toyota Prius NiMH battery requires 10 to 15 kilograms (22 to 33 lb) of lanthanum. As engineers push the technology to increase fuel efficiency, twice that amount of lanthanum could be required per vehicle.
Hydrogen sponge 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. Heat energy is released every time they do so; therefore these alloys have possibilities in energy conservation systems.
Mischmetal, a pyrophoric alloy used in lighter flints, contains 25% to 45% lanthanum. …
Naturally occurring lanthanum is made up of two isotopes, the stable 139La and the primordial long-lived radioisotope 138La. 139La is by far the most abundant, making up 99.911% of natural lanthanum: it is produced in the s-process (slow neutron capture, which occurs in low- to medium-mass stars) and the r-process (rapid neutron capture, which occurs in core-collapse supernovae). It is the only stable isotope of lanthanum. The very rare isotope 138La is one of the few primordial odd–odd nuclei, with a long half-life of 1.03×1011 years. It is one of the proton-rich p-nuclei which cannot be produced in the s- or r-processes. 138La, along with the even rarer 180mTa, is produced in the ν-process, where neutrinos interact with stable nuclei. All other lanthanum isotopes are synthetic: with the exception of 137La, which has a half-life of about 60,000 years, all of them have half-lives less than two days, and most have half-lives less than a minute. The isotopes 139La and 140La occur as fission products of uranium.
Lanthanum has no known biological role in humans. The element is very poorly absorbed after oral administration and when injected its elimination is very slow. Lanthanum carbonate (Fosrenol) was approved as a phosphate binder to absorb excess phosphate in cases of end stage renal disease.
While lanthanum has pharmacological effects on several receptors and ion channels, its specificity for the GABA receptor is unique among trivalent cations. Lanthanum acts at the same modulatory site on the GABA receptor 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.
Lanthanum is a cofactor for the methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV, although the great chemical similarity of the lanthanides means that it may be substituted with cerium, praseodymium, or neodymium without ill effects, and with the smaller samarium, europium, or gadolinium giving no side effects other than slower growth.