Lanthanides

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Photo by: GM

The lanthanides are the chemical elements found in Row 6 of the periodic table between Groups 3 and 4. They follow lanthanum (La), element #57, which accounts for their family name. The lanthanides include the metals cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

Lanthanides as rare earth elements

At one time, the lanthanides were called the rare earth elements. The name suggests that chemists once thought that the elements were present in Earth's crust in only very small amounts. As it turns out, with one exception, that assumption was not correct. (That exception is promethium, which was first discovered in the products of a nuclear fission reaction in 1945. Very small amounts of promethium have also been found in naturally occurring ores of uranium.)

The other lanthanides are relatively abundant in Earth's crust. Cerium, for example, is the twenty-sixth most abundant element. Even thulium, the second rarest lanthanide after promethium, is more abundant than iodine.

The point of interest about the lanthanides, then, is not that they are so rare, but that they are so much alike. Most of the lanthanides occur together in nature, and they are very difficult to separate from each other. Indeed, the discovery of the lanthanide elements is one of the most intriguing detective stories in all of chemistry. That story includes episodes in which one element was thought to be another, two elements were identified as one, some elements were mistakenly identified, and so on. By 1907, however, the confusion had been sorted out, and all of the lanthanides (except promethium) had been identified.

Occurrence

The most important source of the lanthanides is monazite, a heavy dark sand found in Brazil, India, Australia, South Africa, and the United States. The composition of monazite varies depending on its location, but it generally contains about 50 percent of lanthanide compounds by weight. Because of the similarity of their properties and their occurrence together in nature, the lanthanides can be separated from each other and purified only with considerable effort. Consequently, commercial production of the lanthanides tends to be expensive.

Properties

Like most metals, the lanthanides have a bright silvery appearance. Five of the elements (lanthanum, cerium, praseodymium, neodymium, and europium) are very reactive. When exposed to air, they react with oxygen to form an oxide coating that tarnishes the surface. For this reason these metals are stored under mineral oil. The remainder of the lanthanides are not as reactive, and some (gadolinium and lutetium) retain their silvery metallic appearance for a long time.

When contaminated with nonmetals, such as oxygen or nitrogen, the lanthanides become brittle. They also corrode more easily if contaminated with other metals, such as calcium. Their melting points range from about 819°C (1,506°F) for ytterbium to about 1,663°C (3,025°F) for lutetium. The lanthanides form alloys (mixtures) with many other metals, and these alloys exhibit a wide range of physical properties.

The lanthanides react slowly with cold water and more rapidly with hot water to form hydrogen gas. They burn readily in air to form oxides. They also form compounds with many nonmetals, such as hydrogen, fluorine, phosphorous, sulfur, and chlorine.

Uses of lanthanides

Until fairly recently, the lanthanides had relatively few applications; they cost so much to produce that less expensive alternatives were usually available. The best known lanthanide alloy, Auer metal, is a mixture of cerium and iron that produces a spark when struck. It has long been used as a flint in cigarette and gas lighters. Auer metal is one of a series of mixed lanthanide alloys known as misch metals. The misch metals are composed of varying amounts of the lanthanide metals, mostly cerium and smaller amounts of others such as lanthanum, neodymium, and praseodymium. They have been used to impart strength, hardness, and inertness to structural materials. They have also been used to remove oxygen and sulfur impurities from various industrial systems.

In recent years, less expensive methods have been developed for the production of the lanthanides. As a result, they are now used in a greater variety of applications. One such application is as catalysts, substances that speed up chemical reactions. In the refining industry, for example, the lanthanides are used as catalysts in the conversion of crude oil into gasoline, kerosene, diesel and heating oil, and other products.

The lanthanides are also used as phosphors in color television sets. Phosphors are chemicals that glow with various colors when struck by electrons. For example, oxides of europium and yttrium are used to produce the red colors on a television screen. Other lanthanide compounds are used in streetlights, searchlights, and in the high-intensity lighting present in sports stadiums.

The ceramics industry uses lanthanide oxides to color ceramics and glasses. Optical lenses made with lanthanum oxide are used in cameras and binoculars. Compounds of praseodymium and neodymium are used in glass, such as in television screens, to reduce glare. Cerium oxide has been used to polish glass.

The lanthanides also have a variety of nuclear applications. Because they absorb neutrons, they have been employed in control rods used to regulate nuclear reactors. They have also been used as shielding materials and as structural components in reactors. Some lanthanides have unusual magnetic properties. For instance, cobalt-samarium magnets are very strong permanent magnets.

[ See also Element, chemical ; Periodic table ]