Lanthanides

views updated

Lanthanides

The lanthanides are a series of 14 metallic elements that appear at the bottom of the periodic table. Lanthanum, the element preceding the lanthanides in the periodic table, is usually also included in a discussion of the lanthanides since all 15 elements have very similar properties. When first discovered and isolated, the lanthanides were called the rare earth elements. Many uses have been found for these elements and their compounds despite their expense.

Discovery of the lanthanides

Although once called the rare earths, most lanthanides are not particularly rare in Earth’s crust. Today, with the exception of promethium, the lanthanides are known to have abundances comparable to many other elements. The 15 elements, together with their chemical symbols, are lanthanum (La), 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 lute-tium (Lu). Thulium, one of the scarcest lanthanides, has an abundance in Earth’s crust of 0.2 parts per million (ppm), and is more abundant than arsenic or mercury. The most abundant is cerium (46 ppm), which is more abundant than tin. Promethium, which is radioactive, is found only in trace amounts in uranium ores. Small amounts have been isolated from the spent fuel of nuclear reactors. The lanthanide elements, cerium through lutetium, have corresponding atomic numbers of 58 through 71.

The discovery of the lanthanides spanned more than a century of work, beginning in the late 1700s. In 1794, Finnish chemist Johan Gadolin (1760–1852) studied ytterbia, which he believed was a new element. More than a decade later, English chemist Sir Humphry Davy (1778–1829) showed that ytterbia was a compound, composed of oxygen and a metal, rather than an element. Because many of the lanthanides occur together in the same minerals, and due to their similar properties, separation of the lanthanides proved a challenge to nineteenth century chemists. This often led to confusion, since it was difficult to distinguish one element from another or from its mineral precursor. The mid-nineteenth century invention of the spectroscope, an instrument that measures light emission and absorption from heated substances, assisted with unraveling lanthanide identification. With this instrument it is possible to analyze light from the sun and the stars, and chemists now know that lanthanides are present in other parts of our solar system and even beyond it.

Properties of the lanthanides

Like many metals, the lanthanides have a bright silvery appearance. Five of the elements (La, Ce, Pr, Nd, and Eu) are very reactive and when exposed to air 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 (Gd and Lu) retain their silvery metallic appearance for a long time. When contaminated with nonmetals, such as oxygen or nitrogen, the lanthanides become brittle. They will also corrode more easily if contaminated with other metals, such as calcium. Their melting points, which range from about 1,506.2°F (819°C) (Yb) to about 3,025.4°F (1,663°C) (Lu), are also sensitive to contamination. The lanthanides form alloys with many other metals, and these alloys exhibit a wide range of physical properties.

The lanthanides react slowly with cold water (more rapidly with hot water) to form hydrogen gas, and readily burn in air to form oxides. Oxides are substances in which a metal and oxygen have chemically combined to form a compound. For example, samarium and oxygen combine to form the compound samarium oxide. Yttrium has a natural protective oxide coating, making it much more resistant. The lanthanides form compounds with many nonmetals, such as hydrogen, fluorine, phosphorous, sulfur, and chlorine, and heating may be required to induce these reactions.

The arrangement of electrons in an atom (the electron configuration) influences the atom’s reactivity with other substances. In particular, it is the outer or valence electrons-those furthest away from the center of the atom-that are most involved in reactions since these are exposed to the surrounding environment. All the lanthanides, from cerium to lutetium, have a similar arrangement of their outer electrons. This explains why they are all found in nature together and why they all react similarly. When they react with other elements to form compounds, most lanthanides lose three of their outer electrons to form tripositive ions. For most compounds of the lanthanides, this is the most stable ion. Some lanthanides form ions with a positive two or four charge, but these are usually not as stable. A comparison of the sizes of the lanthanide atoms, and their ions, reveals a progressive decrease in going from lanthanum to lutetium and is referred to as the lanthanide contraction. Compounds containing positive and negative ions are called ionic compounds. Most ionic lanthanide compounds are soluble in water. Compounds of lanthanides with the element fluorine (lanthanide fluorides), however, are insoluble. Adding fluoride ions to a solution of tripositive lanthanide ions can generally be used as a characteristic test for the presence of the lanthanides. Likewise, lanthanide oxalates (oxalate is the negative ion C_zO₄<->-2) have low solubility.

Isolation and production

The lanthanides occur naturally in many minerals but are most concentrated in 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 generally contains about 50% of lanthanide compounds by weight. Like any group of elements that have similar properties and that occur in nature together, the separation and purification of the lanthanides requires considerable effort. Consequently, commercial production of the lanthanides tends to be expensive.

To separate the lanthanides from other elements occurring with them, they are chemically combined with specific substances to form lanthanide compounds with low solubility (oxalates and fluorides, for example). A process known as ion exchange is then used to separate the lanthanides from each other. In this process, a solution of the lanthanides in ionic, soluble form is passed down a long column containing a resin. The lanthanide ions “stick” to the resin with various strengths based on their ion size. The lanthanum ion, being smallest, binds most tightly to the resin, whereas the largest ion, lutetium, binds the weakest. The lanthanides are then washed out of the ion exchange column with various solutions, emerging one at a time, and so are separated. Each is then mixed with acid, precipitated as the oxalate compound, and then heated to form the oxide. A number of methods have been used to obtain the lanthanides in metallic form. For example, the oxides can be converted to fluorides or chlorides which are then reduced with calcium to metallic form.

Uses of lanthanides

Although the lanthanide elements, alloys, and compounds have many uses, less expensive alternatives functioning just as efficiently are used where possible. But despite their cost, the unique properties of the lanthanides do sometimes favor their use over cheaper substances, and millions of tons of lanthanides, in metallic, alloy, and compound form, are produced annually. One of the earliest uses involved an alloy of cerium and iron, called Auer metal, which produced a brilliant spark when struck. This has been widely used as a flint in cigarette and gas lighters. Auer metal is one of a series of mixed lanthanide alloys called misch metals that have a variety of metallurgical applications. These alloys 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 systems.

As catalysts (substances that speed up chemical reactions), the lanthanides are widely used in the oil refining industry since they speed up the conversion of crude petroleum into widely used consumer products such as gasoline. The color television industry also makes extensive use of europium and yttrium oxides to produce the red colors on television screens. Other lanthanide compounds are used in streetlights, searchlights, and in the high-intensity lighting 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. Others (Pr and Nd) are used in glass, such as in television screens, to reduce glare. Cerium oxide has been used to polish glass. The lanthanides have a variety of nuclear applications. Because they absorb neutrons, they have been used 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.

Lanthanide

Lanthanum—which is used in various types of optical glass—is the third element in Row 6 of the periodic table. Lanthanum’s atomic number is 57, its atomic mass is 138.9055, and its chemical symbol is La.

Credit for the discovery of lanthanum is usually given to Swedish chemist Carl Gustav Mosander (1797–1858). Mosander was very much interested in an unusual black rock found near the town of Bastnas, Sweden, in the 1830s. Over the next 60 years, chemists discovered seven new elements in that rock, one of them being lanthanum, discovered in 1839. The element was named after the Greek word lanthanein, meaning to hide.

Lanthanum is a white metal that is both ductile and malleable. It is relatively soft and can be cut with a sharp knife. The element’s melting point is about 1,688°F (920°C), its boiling point is approximately 6,267°F (3,464°C), and its density is about 3.55 ounces per cubic inch (6.15 grams per cubic centimeter). It is a solid at room temperature. Lanthanum is chemically very active, reacting with both cold water and most acids. It also reacts with oxygen in moist air.