Lanthanum (La)

• Element Symbol: La
• Atomic Number: 57
• Atomic Mass: 138.9055
• Group # in Periodic Table: n/a
• Group Name: Lanthanides
• Period in Periodic Table: 6
• Block of Periodic Table: f-block (disputed)
• Discovered by: Carl Gustaf Mosander (1839)

Lanthanum is a soft, ductile, silvery-white rare earth element. It is the first element of the lanthanide series. Lanthanum is also considered to be the first element of the sixth-period transition metals. This element is usually found in combination with cerium and other rare earth elements. It oxidizes almost instantly when it is exposed to air. Lanthanum compounds have numerous applications.

Lanthanum was discovered in 1839 in the mineral cerite by Carl Gustaf Mosander at the Karolinska Institutet, Stockholm. Mosander had managed to extract it from cerium, which had been discovered earlier in 1803. In 1839 he happened to notice this new element because while most of his sample of cerium oxide was insoluble, some of it was in fact soluble. From this fact he figured out that he was observing the oxide of a new element. Mosander extracted lanthanum from the cerite by taking finely powdered cerium nitrate and adding it to cold, dilute nitric acid. The result was that some of the cerium nitrate powder dissolved in the acid. This seemed to indicate that a new rare earth element might be present. This new oxide was more basic than cerium oxide. However, unlike cerium oxide, this apparent element actually dissolved in the acid solution. Mosander then separated the solution from the precipitate using a combination of sodium oxalate and heat. The final result was a pale, brick-colored oxide of the new rare earth element. The name "lanthana" for the new oxide was suggested to Mosander by his mentor, J. J. Berzelius. The name is derived from the Greek word lanthano, which means "to be hidden."

Coincidentally, another student at the institute, Axel Erdmann, discovered lanthanum in the same year. His source was a new mineral from Låven Island, which is located in a Norwegian fjord. Conceding that he had come in second in the race that resulted in this almost simultaneous discovery, Erdmann graciously called the new mineral mosandrite in honor of Mosander.

Mosander continued to study lanthana. Later on, in 1841 he announced the discovery of another rare earth element contained within lanthanum. He named it didymium. Mosander did so because the substance seemed to be "an inseparable twin brother of lanthanum" (Weeks 704). Pure lanthanum metal was first produced in 1923 by H. Kremers and R. Stevens, who were American chemists.

Physical Properties

Lanthanum is a silvery-white metal that is so soft that it can be cut with a knife. This element’s softness is explained by its hardness rating on the Mohs scale, which is only 2.5. It is also ductile and malleable because it is not very dense. Its density at room temperature is only 6.16 grams per cubic centimeter (g/cm³). By contrast, the density of platinum is 21.09 g/cm³. Lanthanum’s standard state at 298 kelvins (K) is solid. It has a high melting point of 920 degrees Celsius (°C) and a high boiling point of 3464 °C. The specific heat of lanthanum is 0.19 joules per kilogram-kelvin (J/kg·K). Its thermal conductivity is 13.5 watts per meter-kelvin (W/m·K). Its electrical conductivity is 1.6 × 10⁶ siemens per meter (S/m).

Chemical Properties

Lanthanum has two oxidation states, +3 and +2, meaning that it is exceptionally electropositive. This characteristic makes the element extremely chemically reactive. Lanthanum is so easily oxidized, in fact, that any exposed surface tarnishes almost instantly when exposed to air, and a centimeter-sized sample of the element can be totally oxidized in under a year. When it reacts with water, the products are lanthanum hydroxide and hydrogen gas. Lanthanum also forms binary compounds with the halogens, such as chlorine and iodine, as well as with arsenic, boron, carbon, nitrogen, selenium, silicon, sulfur, and phosphorus.

Naturally occurring lanthanum is made up of two isotopes, one stable (lathanum-139) and one radioactive (lanthanum-138). So far, thirty-eight radioisotopes have been identified. The most stable radioisotope is lanthanum-138, which has a half-life of 1.05 × 10¹¹ years. The second-most stable radioisotope, lanthanum-137, has a half-life of sixty thousand years. The thirty-six other radioisotopes have half-lives ranging from less than twenty-four hours to less than one minute.

Applications

Despite the fact that lanthanum belongs to the rare earth metals group, it is actually not rare at all. The so-called rare earth elements earned this name for one simple reason: these elements were indeed rare when compared to the common earth elements, such as lime or magnesia. Historically, there were only a few known deposits of the rare earth metals. Lanthanum is in the same group as the other rare earth metals because the process required to mine it is incredibly difficult and expensive. Lanthanum is available in relatively high concentrations of 32–34 parts per million in Earth’s crust. Conversely, it is in fact rare throughout the solar system. Lanthanum’s concentration in meteorites and planets is only 2 parts per billion.

Lanthanum is also considered rare because it is not found free in nature. Primarily, it is found in two minerals, monazite and bastnaesite. Generally, the concentration of lanthanum is higher in bastnaesite than it is in monazite. Until 1949, bastaesite was considered to be an obscure mineral. It was not at all thought of as a potential commercial source for the lanthanides. However, a large carbonatite mineral deposit was discovered at the California Mountain Pass Rare Earth Mine in that year. Bastnaesite is a type of carbonatite mineral ore. Soon after, other bastnaesite sources were discovered in Africa and China. Commercially, lanthanum is recovered from bastnaesite and monazite using a variety of complex extraction techniques, including ion exchange and solvent extraction.

Historically, lanthanum was first used in gas lantern mantles. A gas mantle, or Welsbach mantle, was a device that generated a bright white light when heated by a flame. The device’s name refers to its original heat source—the gaslights that used to fill the streets of Europe and North America in the late nineteenth century, before the use of electrical lights become common. The term "mantle" refers to the way this device was hung above the flame. The inventor, Carl Auer von Welsbach, used a mixture he called "actinophor," and he patented the device in 1885. The mixture consisted of 60 percent magnesium oxide, 20 percent lanthanum oxide, and 20 percent yttrium oxide.

As for its modern-day uses, lanthanum metal has absolutely no commercial value. However, its alloys are in fact used in a variety of commercial products. The most relevant ones to today’s world include hybrid car batteries, cigarette lighter flints, night-vision goggles, and movie lighting.

Lanthanum alloys are also used in the medical and scientific world. Medically, lanthanum carbonate has been approved as a medication for the treatment of kidney failure. Specifically, it is used to reduce the blood levels of phosphate in patients with this debilitating condition. Scientifically, certain lanthanum compounds have been used in a similar fashion. Some pool-cleaning products use these compounds to reduce the levels of phosphate nutrients that algae prefer to feed on.

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