LANTHANUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Lanthanum is the third element in Row 6 of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Lanthanum is a transition element in Group 3 (IIIB) of the periodic table. Lanthanum's position makes it one of the transition metals. The transition metals are found in the center of the periodic table, in Groups 3 through 12.

Lanthanum can also be classified as a rare earth element. The rare earth elements are the 15 elements that make up Row 6 of the periodic table. That group of elements is also called the lanthanides. Either way of classifying Lanthanum is acceptable to most chemists.

SYMBOL
La

ATOMIC NUMBER
57

ATOMIC MASS
138.9055

FAMILY
Group 3 (IIIB)
Transition metal

PRONUNCIATION
LAN-tha-num

Lanthanum was first discovered by Swedish chemist Carl Gustav Mosander (1797-1858) in 1839. This discovery was the first chapter in a long and interesting story. At the end of that story, six more elements had been discovered. All of these elements occur together in nature and are hard to separate from each other.

Lanthanum metal has relatively few uses. Some of its compounds however, are used in lamps, color television sets, cigarette lighters, and optical fibers.

Discovery and naming

Toward the end of the 1830s, Mosander became interested in an unusual black stone found near the town of Bastnas, Sweden. He found that the stone contained two new materials. He thought those materials were both new elements. Mosander called them cerium and lanthanum. He was right about cerium, but wrong about lanthanum. The material Mosander called lanthanum later turned out to be a mixture of six new elements.

It took scientists more than 60 years to sort out these elements and separate them from each other. It was not until 1923 that a pure sample of lanthanum metal was even prepared. Still, Mosander is given credit for the discovery of lanthanum.

Physical properties

Lanthanum is a white, ductile, malleable metal. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets. It is relatively soft and can be cut with a sharp knife. Its melting point is 920°C (1,690°F) and its boiling point is 3,454°C (6,249°F). Its density is about 6.18 grams per cubic centimeter.

Chemical properties

Lanthanum is a very active metal. It reacts with most acids and with cold water, although slowly. With hot water, it reacts more quickly:

Lanthanum also reacts with oxygen in the air, especially if the air is moist.

Occurrence in nature

Lanthanum is relatively common in the Earth's crust. Its abundance is thought to be as high as 18 parts per million. That would make it nearly as common as copper or zinc . Unlike those metals, however, it usually does not occur in one place, as in copper mines. Instead, it is spread widely throughout the Earth's crust. Its most common minerals are monazite, bastnasite, and cerite. These minerals generally contain all the other rare earth elements as well.

Isotopes

Two naturally occurring isotopes of lanthanum are known. They are lanthanum-138 and lanthanum-139. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Lanthanum-138 is very rare and is radioactive. Its half life is about 100 billion years. The half life of a radioactive element is the time it takes for half of a sample of the element to break down. Only 5 grams of a 10-gram sample of lanthanum-138 will remain after 100 billion years. The other 5 grams would have broken down to form a new isotope.

More than a dozen artificial radioactive isotopes have also been made. These isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive. None of the radioactive isotopes of lanthanum have any commercial use.

Extraction

The rare earth elements are very similar to each other. Separating them is a very difficult task. The ores are first treated with sulfuric acid (H₂SO₄). The materials produced are then passed through a series of steps and the individual elements separated from each other.

Uses and compounds

One of the most important uses of lanthanum compounds is in carbon arc lamps. In a carbon arc lamp, an electrical current is passed through the lamp electrode. The electrode is made of carbon and traces of other materials that have been added. The current causes the carbon to heat up and give off a brilliant white light. The exact color of the light depends on the other materials that have been added to the carbon. Lanthanum fluoride (LaF₃) and lanthanum oxide (La₂O₃) are usually used for this purpose.

These two compounds are also used in making phosphors. A phosphor is a material that gives off light when struck by electrons. The color of the phosphor depends on what elements are present. The colors produced in a color television set are caused by phosphors painted on the back of the screen.

Compounds of lanthanum are also used to make special kinds of glass. High quality lenses, for example, are often made of glass containing a small amount of lanthanum.

One of the oldest uses of lanthanum metals is in the production of misch metal. Misch metal is an alloy that produces sparks when struck. One application of misch metal is in the manufacture of cigarette lighters.

Lanthanum oxide is used to make phosphors. Phosphors give off light when struck by electrons. They produce the colors on television sets.

A new application of lanthanum glass is in making optical fibers. An optical fiber is a wire-like material made of glass. It carries light in the same way a copper wire carries electricity. Optical fibers are becoming more popular as methods for carrying audio, video, and digital messages. In many cases, optical fibers have replaced copper wires for this purpose.

In 1974, a team of French researchers invented a product called ZBLAN. ZBLAN is made of zirconium, barium, lanthanum, aluminum, and sodium. ("ZBLAN" comes from the first letters of each element's symbol.) Scientists have found that ZBLAN is 100 times better at carrying messages than traditional optical fibers.

Health effects

Lanthanum and its compounds are poisonous in high concentrations. They should be handled with care.

Lanthanum

melting point: 920°C
boiling point: 3,469°C
density: 6.16 g/cm ³
most common ions: La ³⁺

Elemental lanthanum has a ground state (electronic configuration) of [Xe]5d6s². Naturally occurring lanthanum is a mixture of two stable isotopes , ¹³⁸La and ¹³⁹La. The element was discovered in 1839 by Carl Gustaf Mosander in the form of the lanthanum oxide, at that time called "lanthana." The name is derived from the Greek lanthanein ("to lie hidden"), as the element had been overlooked due to its similarity to the earlier discovered cerium.

Monazite and bastnasite are the principal lanthanum ores, in which lanthanum occurs together with other members of the rare earth elements or the lanthanides . It can be separated from the other rare earths by ion exchange or solvent extraction techniques. Lanthanum is a silver-white, malleable, and ductile metal . It is soft enough to be cut with a knife. The metal is rapidly oxidized when exposed to air. Cold water attacks lanthanum only slowly, but reaction with hot water is fast.

Lanthanum chemistry is dominated by the trivalent lanthanum(III) ion, La³⁺. This ion forms ionic bonds with ligands containing an oxygen or nitrogen donor atom. The ground state electronic configuration of La³⁺ is [Xe]4f⁰. Due to the absence of unpaired 4f electrons, lanthanum(III) compounds are colorless, both in solution and in the solid state. Lanthanum(III) oxide is added to optical glass to increase its refractive index and alkali resistance. Lanthanum-nickel alloys are being used in the storage of hydrogen gas. Thulium(III)-doped lanthanum oxybromide (LaOBr:Tm³⁺) is a blue-emitting phosphor used in x-ray intensifying screens.

see also Cerium; Dysprosium; Erbium; Europium; Gadolinium; Holmium; Lanthanides; Lutetium; Neodymium; Praseodymium; Promethium; Samarium; Terbium; Ytterbium.

Koen Binnemans

Bibliography

Cotton, Simon (1991). Lanthanides and Actinides. New York: Oxford University Press.

Kaltsoyannis, Nikolas, and Scott, Peter (1999). The f Elements. New York: Oxford University Press.

lanthanum (symbol La) Silvery-white metallic element of the lanthanide series, first identified in 1839. Its chief ores are monazite and bastnasite. Soft, malleable, and ductile, lanthanum is used as a catalyst in cracking crude oil, in alloys and to manufacture optical glasses. Properties: at.no. 57; r.a.m. 138.9055; r.d. 6.17; m.p. 920°C (1688°F); b.p. 3454°C (6249°F); most common isotope La¹³⁹ (99.91%).

lan·tha·num / ˈlan[unvoicedth]ənəm/ • n. the chemical element of atomic number 57, a silvery-white rare earth metal. (Symbol: La)

lanthanum (chem.) rare metallic element. XIX. modL., f. Gr. lanthánein escape the notice of (from its having remained undetected in cerium oxide).