Lanthanum
In 1839, the Swedish chemist Carl Gustaf Mosander examined a sample of cerium nitrate. He roasted it in air and treated the resulting oxide with dilute nitric acid. This process revealed an impurity that had been hiding within the known element. Mosander named this new substance lanthana from the ancient Greek word meaning to lie hidden. The name reflected how difficult it was to separate from its neighbor cerium. Another discovery occurred that same year when Axel Erdmann found lanthanum in a mineral from Låven island in Norway. Pure metal remained elusive for decades because the separation process proved incredibly complex. It took until 1923 before scientists finally isolated pure lanthanum metal.
A single gas-phase atom of lanthanum holds fifty-seven electrons arranged in the configuration [Xe]5d6s. These three valence electrons sit outside the noble gas core of xenon. In chemical reactions, the atom almost always gives up these three electrons to form the +3 oxidation state. Unlike later elements in the series, lanthanum has no 4f electrons as a single atom. This absence makes it only very weakly paramagnetic compared to strongly paramagnetic neighbors like neodymium. The melting point reaches 920 degrees Celsius, which is the second-lowest among trivalent lanthanides. At room temperature, the metal adopts a hexagonal crystal structure labeled alpha-lanthanum. When heated to 310 degrees Celsius, it shifts to a face-centered cubic structure. Further heating to 865 degrees Celsius changes it again to a body-centered cubic structure. The metal itself remains soft and tarnishes quite rapidly when exposed to air.
Naturally occurring lanthanum consists of two isotopes found in the Earth's crust. The stable isotope lanthanum-139 makes up 99.911 percent of all natural lanthanum. Scientists produce this isotope through the s-process inside low- to medium-mass stars. It also forms during the r-process within core-collapse supernovae. A rare primordial radioisotope exists called lanthanum-138 with a half-life of 100 million years. This odd-odd nucleus cannot be created by standard stellar processes. Instead, it appears via the nu-process where neutrinos interact with stable nuclei. All other known isotopes are synthetic and short-lived. Most have half-lives under one minute while others last less than two days. Two specific isotopes occur as fission products of uranium. The very long-lived lanthanum-138 stands out as one of the few primordial odd-odd nuclei known to science.
Lanthanum possesses the largest atomic radius among all lanthanides. Consequently, it reacts more vigorously than its neighbors. A centimeter-sized sample will corrode completely within a year as its oxide flakes off like iron rust. The metal burns readily to form lanthanum trioxide which is almost as basic as calcium oxide. At room temperature, it reacts with halogens to create trihalides. Warming allows formation of binary compounds with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon, and arsenic. Reaction with water produces lanthanum hydroxide accompanied by heat evolution and a hissing sound. In dilute sulfuric acid, the element forms the aquated tripositive ion. This ion remains colorless in aqueous solution because it lacks d or f electrons. Some lanthanum plus two compounds exist but they remain much less stable than their trivalent counterparts. Coordination chemistry stays limited due to large ionic radius and great electropositivity.
Mining operations target minerals such as monazite and bastnäsite where lanthanum composes about a quarter of the lanthanide content. Bastnäsite usually lacks thorium making purification of light lanthanides less involved. Crushed ore undergoes treatment with hot concentrated sulfuric acid releasing carbon dioxide and hydrogen fluoride. The product then dries before leaching with water leaves early lanthanide ions including lanthanum in solution. Monazite requires more complex steps involving repeated electromagnetic separation due to magnetic properties. Acid treatment creates water-soluble sulfates followed by partial neutralization with sodium hydroxide to pH three to four. Thorium precipitates out as hydroxide and gets removed. Ammonium oxalate converts rare earths into insoluble oxalates which anneal into oxides. Nitric acid dissolves these oxides excluding cerium whose oxide remains insoluble. Lanthanum separates as a double salt with ammonium nitrate through crystallization. Pure metal finally emerges from heating its oxide with ammonium chloride or fluoride at temperatures between 300 and 400 degrees Celsius. Reduction follows using alkali or alkaline-earth metals in vacuum or argon atmosphere.
Carl Auer von Welsbach patented gas lantern mantles in 1886 using a mixture of lanthanum oxide and zirconium oxide. Modern nickel-metal hydride batteries utilize mischmetal containing over fifty percent lanthanum for the anode material. The 2008 Toyota Prius model required ten kilograms of lanthanum per battery pack. Hydrogen sponge alloys can store up to four hundred times their own volume of hydrogen gas reversibly. Mischmetal flints contain twenty-five to forty-five percent lanthanum for use in lighters. Hot cathode materials in electronic vacuum tubes rely on lanthanum compounds for strong electron emissivity. Heavy fluoride glass named ZBLAN uses lanthanum trifluoride for superior infrared transmittance in fiber-optical systems. Scintillators made from cerium-doped lanthanum bromide detect neutrons or gamma rays efficiently. Carbon arc lamps consumed about twenty-five percent of rare-earth compounds until phase-out. Special optical glasses improve alkali resistance while camera lenses benefit from high refractive index properties. Lanthanum carbonate treats hyperphosphatemia seen in end-stage kidney disease as phosphate binders.
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Common questions
Who discovered lanthanum and when was it found?
Swedish chemist Carl Gustaf Mosander discovered lanthanum in 1839 while examining cerium nitrate. Another discovery occurred that same year when Axel Erdmann found the element in a mineral from Låven island in Norway.
When was pure lanthanum metal first isolated by scientists?
Scientists finally isolated pure lanthanum metal in 1923 after decades of complex separation processes. Pure metal remained elusive for many years following its initial discovery in 1839.
What is the natural abundance of stable lanthanum-139 isotopes on Earth?
The stable isotope lanthanum-139 makes up 99.911 percent of all natural lanthanum found in the Earth's crust. Scientists produce this isotope through the s-process inside low to medium mass stars and during the r process within core collapse supernovae.
How does lanthanum react with water at room temperature?
Reaction with water produces lanthanum hydroxide accompanied by heat evolution and a hissing sound. The metal corrodes completely within a year as its oxide flakes off like iron rust when exposed to air.
Which minerals are mined to extract lanthanum for industrial use?
Mining operations target minerals such as monazite and bastnäsite where lanthanum composes about a quarter of the lanthanide content. Bastnäsite usually lacks thorium making purification of light lanthanides less involved compared to other sources.