Lanthanum

Lanthanum is a chemical element on the periodic table whose symbol is La and its atomic number is 57.

It is the first element of the lanthanide series.

History

It was discovered by the Swedish chemist Carl Gustaf Mosander in 1839. It owes its name to the Greek verb lanthaneîn (λανθανεῖν), which means "hidden", since the metal was "hidden" in a cerium ore. Mosander discovered the element lanthanum in impure cerium nitrate. He was extracted from the ground (oxide insoluble in water) lanthanum (lanthanum oxide), treating it with a strong acid. (Other lanthanide elements were discovered in impurities of yttrium and cerium minerals.)

Features

Lanthanum is the first element and prototype of the lanthanide series. On the periodic table, it appears to the right of the alkaline earth metal barium and to the left of the lanthanide cerium. Lanthanum is often considered a group 3 element, along with scandium, yttrium, and actinium, the heavier congener of lanthanum, although this classification is controversial. The 57 electrons of a lanthanum atom are arranged in the [Xe]5d¹6s² configuration, with three valence electrons outside the noble gas nucleus. 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 earlier noble gas xenon. Some lanthanum compounds are also known. (II), but they are much less stable.

Among the lanthanides, lanthanum is exceptional in that it does not have 4f electrons as a single atom in the gas phase. It is therefore only very weakly paramagnetic, unlike the later strongly paramagnetic lanthanides (with the exception of the latter two, ytterbium and lutetium, where the 4f shell is completely filled). However, the lanthanum 4f shell can partially occupying itself in chemical environments and participating in chemical bonds, which is why it is sometimes considered not to be a group 3 element. For example, the melting points of the trivalent lanthanides are related to the degree of electron hybridization 6s, 5d, and 4f (decreasing with increasing share of 4f), and lanthanum has the second lowest melting point (after cerium) among all lanthanides: 920 °C. Which the series goes through: Unsurprisingly, lanthanum is a soft metal. Lanthanum has a relatively high resistivity of 615 nΩm at room temperature; by comparison, the value of the good conductor of aluminum is only 26.50 nΩm. Lanthanum is the least volatile of the lanthanides. Like most lanthanides, lanthanum has a hexagonal crystal structure at room temperature. At 310 °C, lanthanum changes to a face-centered cubic structure, and at 865 °C, it changes to a body-centered cubic structure.

Chemical characteristics

As expected from periodic trends, lanthanum has the largest atomic radius of the lanthanides. Therefore, it is the most reactive among them, clouding rather quickly in air, turning completely dark after several hours, and can easily burn to form lanthanum(III) oxide, La₂ O₃, which is almost as basic as calcium oxide. A few centimeter-sized sample of lanthanum corrodes completely in a year as its oxide crusts off as iron oxide, in rather than forming a protective oxide coating, such as aluminium, scandium, and yttrium. Lanthanum reacts with halogens at room temperature to form the trihalides, and on heating will form binary compounds with the nonmetals nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon, and arsenic. Lanthanum reacts slowly with water to form lanthanum(III) hydroxide, La(OH)₃. In dilute sulfuric acid, lanthanum readily forms the aqueous trippositive ion [La(H₂O)_9]³⁺: This is colorless in aqueous solution since La³⁺ has no d or f electrons. Lanthanum is the strongest base and lasts among the rare earth elements, again expected to be the largest of them.

Isotopes

Extract of the nucleid graph showing stable isotopes (black) from barium (Z = 56) to neodymium (Z = 60).

Natural lanthanum is composed of two isotopes, the stable La¹³⁹ and the long-lived radioisotope ¹³⁸. La¹³⁹ is by far the most abundant, up to 99.910% 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 of supernovae). The very rare isotope ¹³⁸La is one of the few primordial odd nuclei with a long half-life of 1.05 × 10¹¹ years. It is one of the proton-rich p-nuclei that cannot be produced in s or r processes. La¹³⁸, along with the even rarer La138, is produced in the ν process, where neutrinos interact with stable nuclei. All other lanthanum isotopes are synthetic: with the exception of La¹³⁷ with a half-life of about 60,000 years, all of them have half-lives of less than one day and most have half-lives of less than one minute. The isotopes La¹³⁹ and La¹⁴⁰ are produced as fission products of uranium.

Methods of production and chemistry

Lanthanum, like the other rare earths, exists only in minerals because of its chemical reactivity.

By reducing anhydrous lanthanum fluoride with calcium:

2LaF₃ + 3Ca → 3CaF₂ + 2La

The solidification temperature of lanthanum is 900 °C.

Special applications

• Alloyed with cerio, neodymium, praseodymium, gadolinium and iterbio forms the alloy called ferrocerio, used to manufacture litter stones.
• Lantano oxide gives the glass resistance to the bases and is used for the manufacture of special optical glass.

• Hydrogen sponges are being produced with alloys containing woolen. These alloys allow up to 400 times their gas volume and the process is reversible. Every time they take gas, heat energy is released, so they have the possibility of becoming energy conservation systems.
• It is used as a component of the X-ray intensifier screens.
• Lantano carbonate is used in the treatment of Chronic Renal Insufficiency by its ability to form insoluble complexes with phosphates, thus reducing hyperphosphemy.

Biological paper

Lanthanum has no known biological role in humans. This 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 by the FDA in the United States.

Although lanthanum has pharmacological effects at various 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 La³⁺ cation is a positive allosteric modulator at native and recombinant GABA receptors, increasing channel-open time and decreasing desensitization in a subunit configuration-dependent manner.

Lanthanum is an essential cofactor for the enzyme methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV, although the great chemical similarity of the lanthanides means that it can be substituted with cerium, praseodymium or neodymium without effect noxious, and with the smallest samarium, europium, or gadolinium which produce no side effects other than slower growth.

Precautions

Lanthanum has a low to moderate level of toxicity and should be handled with care. The injection of lanthanum solutions causes hyperglycemia, arterial hypotension, spleen degeneration, and liver disorders.^{[citation needed]} The application in carbon arc light led to the exposure of people to oxides and fluorides of rare earth elements, sometimes leading to pneumoconiosis. As the La³⁺ ion is similar in size to the Ca²⁺ ion, is sometimes used as a readily traceable substitute for the latter in medical studies. Lanthanum, like the other lanthanides, is known to affect human metabolism, reducing cholesterol levels, blood pressure, appetite, and risk of blood coagulation. When injected into the brain, it acts as a pain reliever, similar to morphine and other opiates, although the mechanism behind this is still unknown.

Additional bibliography

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