Lanthanides

Type of physical science: Chemistry

Field of study: Chemistry of the elements

The lanthanides are a group of fifteen elements (atomic numbers 57 to 71) formed from the progressive filling of inner f electron shells. They have very similar chemical properties as a result of their +3 charges, and they progressively decrease in size with increasing atomic number. Their magnetic and light-emitting properties have resulted in their use in high-quality magnets and as colored phosphors in television sets.

Overview

The lanthanides, or rare-earth elements, are a group of fifteen elements whose chemical similarities result from the sequential filling of inner electrons with increasing atomic number. This atomic number, which represents the number of protons in the nucleus, determines the kind of element. The fifteen elements are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

The atomic numbers of the lanthanides range from 57 to 71. As the number of protons increases across the row of the lanthanides, electrons are added only to inner f electron shells, so that the outer electrons are progressively pulled inward by this increasingly positively charged nucleus and the radii of the lanthanides gradually become smaller. For example, with a +3 charge, the radius of the lanthanide with the lowest atomic number, lanthanum, is 1.06 angstroms, while the radius of the lanthanide with the highest atomic number, lutetium, is 0.85 angstrom. This gradual decrease in size with increasing atomic number is called the lanthanide contraction.

The lanthanides commonly lose three outer electrons in chemical reactions to form +3-charged ions from the previously neutral atoms. This loss of electrons is called oxidation. Thus, the most common compounds of the oxidized lanthanides have +3 charges on the lanthanides; for example, lanthanum chloride has a 1:3 ratio of lanthanum to chlorine. Cerium, praseodymium, and terbium may also lose a total of four electrons in chemical oxidation and form +4-charged ions in chemical compounds, such as cerium fluoride, which has a 1:4 ratio of cerium to fluorine. Similarly, samarium, europium, and ytterbium may lose a total of only two electrons from the elemental form and form +2-charged ions in compounds, as in europium chloride, with a 1:2 ratio of europium to chlorine.

The lanthanides in their elemental form occur as metals with no charge, and they have a strong tendency to lose their electrons and react vigorously with materials that readily accept electrons. The lanthanide metals, for example, react rapidly in acid solutions (water-based solutions with abundant hydrogen ions) at room temperature to produce lanthanide ions with +3 charges in the solution and release hydrogen gas. The metals react more slowly with neutralized water with no acid to produce hydrogen gas and lanthanide oxides or hydroxides (ions with oxygen and hydrogen), with the lanthanide atoms combining chemically with the oxygen atoms or hydroxide groups in a 1:3 ratio. The speed of this reaction rapidly increases at higher temperatures. Europium reacts the most vigorously of any of the lanthanides with water. The lanthanide metals react slowly with air at room temperature, but they burn readily in air at higher temperatures, about 150 degrees Celsius, to produce rare-earth oxides with rare-earth and oxygen atoms in a 2:3 ratio.

Oxidation of the lanthanides occurs easily in water solutions, but the reverse process—reduction, or the addition of electrons to positively charged ions to form neutral atoms—does not occur readily. Lanthanide ions can be reduced by hydrogen gas to produce the metallic elements with no charge. If lanthanide salts of the halogens are melted and a direct electrical current is passed through them, the positive lanthanide ions are attracted to the negative electrode, where they are reduced by picking up electrons, and then are plated out on the electrode as neutral metals.

The lanthanide atoms with the common +3 charge form compounds with all simple, and many complex, negatively charged ions, or anions. These lanthanides combine with anions such as nitrate (nitrogen and oxygen in a 1:3 ratio), sulfate (sulfur and oxygen in a 1:4 ratio), or carbonate (carbon and oxygen in a 1:3 ratio) to form complexes that are unstable on heating and break down into lanthanide oxides. When lanthanides combine with anions such as halogens or phosphate (phosphorus and oxygen in a 1:4 ratio), they are more stable on heating and melt only at high temperatures. These melted lanthanide halides or phosphates are good electrical conductors, suggesting that they consist mostly of charged particles. Lanthanide halides also conduct electricity very well when dissolved in water. The tendency to produce ions in water solutions decreases from the largest lanthanide, lanthanum, to the smallest, lutetium, apparently because of the great density of charge in the smaller ions and, hence, the greater attraction to the negatively charged ions with which they are bonded.

Lanthanides with +3 charges form compounds with the negative ions of most halogens, perchlorate (chlorine and oxygen in a 1:4 ratio), acetate (negative ion from acetic acid), and nitrate that are soluble in water, meaning they dissolve readily. The lanthanides form compounds with fluoride, chromate (chromium and oxygen in a 1:4 ratio), carbonate, oxygen, hydroxide, phosphate, and oxalate that are relatively insoluble in water. The ease of dissolution of each lanthanide in water with a given negative ion does not necessarily vary in a clear, systematic way with the radii of the lanthanides. For example, lanthanides combined with hydroxide do increase in solubility in order of increasing radius, but the solubility of lanthanides combined with sulfate varies in an erratic fashion. The low solubility of lanthanides combined with the negative oxalate ion in acidic water is important, as it allows for the removal of the lanthanides from most other positively charged ions in water solutions during chemical separation. Other lanthanide compounds that do not readily dissolve in water, such as those combined with oxygen or hydroxide, may dissolve more readily if the water is acidic.

In addition to bonding with simple negative ions to form neutral compounds, the lanthanides also have some tendency to bond with species to form charged complexes called complex ions. For example, they may bond with oxalate to form a lanthanide oxalate, an entire unit of which has a +1 charge; they may bond with citrate (carbon, oxygen, and hydrogen in a 6:7:5 ratio) to form lanthanide citrate, with a -3 charge. This tendency to form complex ions is somewhat similar to the tendency of the alkaline-earth elements, those in the second group in the periodic table, to do the same thing, though the tendency is greater for the transition elements, those in the middle of the periodic table that are filling inner d electron shells.

Lanthanides may also form compounds in which they have +2 or +4 charges. Only the lanthanide compounds of +2-charged samarium, europium, and ytterbium that are insoluble in water are easily prepared. For example, samarium, europium, and ytterbium sulfates, carbonates, and fluorides are not very soluble in water, and they will not be oxidized to the +3 ions of the lanthanides in water. In contrast, only +2-charged europium in europium chloride will not easily be oxidized in water, but the +2-charged samarium and ytterbium chlorides will be oxidized by water. All the other +2-charged lanthanides are easily formed only as lanthanide halides, but they are all oxidized to the +3 ions when placed in water.

Cerium is the lanthanide that most easily forms compounds with +4 charges, but praseodymium and terbium can do so as well. The lanthanide compounds in which the lanthanides have +4 charges are most commonly oxides and fluorides. Lanthanide compounds can be prepared by the oxidation of +3-charged lanthanides with oxygen or fluorine at high temperature to form the +4-charged lanthanide oxides or fluorides, respectively. Cerium hydroxide, in which the cerium has a +3 charge, is so easily oxidized that it can form the +4-charged cerium oxide when in contact with the atmosphere at ordinary temperature. The reaction takes place even more readily in a solution with abundant hydroxide ions relative to hydrogen ions.

Applications

Many of the applications of the lanthanides are a result of magnetic and light-absorption or -emission properties caused by inner f electrons that are not paired with another electron. There are seven inner orbitals in f energy levels, each of which may contain up to two electrons. If an orbital contains only one electron, then it is said to be unpaired. Unpaired electrons can be thought of as tiny magnets, since they are spinning negative particles. This results in the atoms being aligned parallel to a magnetic field, producing what is called paramagnetism. Most of the lanthanides have one or more unpaired electrons and are thus paramagnetic. Dysprosium and holmium have the highest paramagnetic behavior of any of the lanthanides. If all the electrons pair up with one another in inner f orbitals, then they will align themselves in opposition to magnetic fields, displaying diamagnetic behavior. The only diamagnetic lanthanides are lanthanum and lutetium with +3 charges, ytterbium with a +2 charge, and cerium with a +4 charge.

Some lanthanides can be alloyed with some transition elements to produce affordable magnets for some applications. Samarium and neodymium have been alloyed with magnetic transition elements such as cobalt and iron to produce desirable magnets. The transition elements produce most of the magnetism; while the lanthanides may add to the total magnetism, they mainly contribute other desirable properties not obtainable from the transition elements alone. Lanthanide magnets have been used in electroacoustics (for example, in high-quality stereo speakers), electromechanical actuators (in computer printers), electric motors (in watches, machine tools, and auto starters), electric generators (in auto alternators and jet ignitions), electric switches (in thermostats), mechanical torque devices (in brakes, magnetic bearings, and magnetic locks), electron beam control (in particle accelerators), and medical applications (in catheters).

All the +3 ions of the lanthanides, except for those of lanthanum and lutetium, absorb certain regions of the visible-light spectrum or the adjacent wavelength regions and thus may be colored. Absorption of the lower wavelengths of visible light, for example, will produce the red color of neodymium ions with +3 charges. The absorption bands of the lanthanides may be so specific in visible light that they can be used in very special applications, such as making goggles for glass blowers out of neodymium and praseodymium, which strongly absorb the intense yellow light emitted by hot glass. Certain optical filters also may use various lanthanides that absorb light of specific wavelengths.

Lanthanides may also be added to glasses and ceramic materials as coloring agents. For example, neodymium and praseodymium oxides used for color in glass or glazes. Neodymium oxide tends to counteract the green color produced by iron in glass, and praseodymium oxide mixed with zirconium produces a yellow color in ceramic glazes.

The lanthanides may emit visible light if incorporated with another substance such as zinc sulfide and subjected to an external energy source such as heat, electrical current, or magnetic field. These materials have been used as phosphors in color television, fluorescent lamps, and lasers.

Lanthanides can be added to steel or cast iron in order to improve its quality or enhance certain features. They may be used to improve the metal's capacity to be drawn out into thin wires, remove the sulfur from it more easily, improve its fatigue behavior, or improve the high-temperature strength of the steel.

Lanthanides have numerous uses in consumer electronics and related products. Lanthanum in particular is commonly used in the nickel-metal hydride batteries in hybrid cars, while magnets made of neodymium alloy keep their electric motors running. Phosphorescent europium is the source of the anti-counterfeiting luminescence in euro banknotes and can also be used to produce energy-efficient white light-emitting diodes (LEDs).

Finally, some lanthanides have been used to speed up some chemical reactions. Lanthanide oxides have been added to reactions in which hydrogen is being added to or removed from some organic compounds. Some lanthanide chlorides or phosphates have also been used to break down complex organic compounds in petroleum into products such as gasoline.

Context

The similarity in chemical behavior of the lanthanides makes it exceedingly difficult to separate and identify the individual elements. Elements with significantly different properties may be separated chemically in one step, but the lanthanides are so chemically similar that a single-step reaction will only slightly alter the ratio of the lanthanide abundances. Thus, the same reactions must be carried out many hundreds or even thousands of times before a given lanthanide element is purified. In the late eighteenth century, the lanthanide oxides of lower atomic number were separated as a group from a mineral called cerite; the lanthanide oxides of higher atomic numbers were separated at about the same time from a mineral called gadolinite. At the time, it was believed that these two groups of separated oxides were single elements. It took more than a century of rather blind experimentation to unravel the fifteen separate elements that compose this group. One of the biggest problems in much of the nineteenth century was the lack of any experimental means to identify the pure lanthanide elements; workers had no idea whether, in their trial-and-error separations, they had indeed obtained a pure-element lanthanide oxide or whether it was still a mixture of elements. Only when emission and absorption spectroscopy were developed in the early twentieth century could the individual lanthanides be positively separated and identified.

Minerals that concentrate the lanthanides tend to concentrate either those with lower atomic numbers, as do cerite and bastnaesite, or those with higher atomic numbers, as do xenotime, gadolinite, and euxenite. Monazite may contain a wider range of lanthanides. Geologic processes can help concentrate some minerals. For example, monazite is often concentrated in beach sands by wave action. It may then be concentrated by physical means, such as settling in a liquid of appropriate density to allow the monazite to sink and most of the other minerals to float.

The lanthanide-rich minerals must be chemically decomposed and the lanthanides separated as a group. Often the element thorium is present as an impurity, but it can be separated chemically from the lanthanides. The individual lanthanide elements often must then be separated by a tedious chemical method that uses a long series of reactions to purify a given element gradually. For example, monazite may be decomposed in concentrated sulfuric acid to produce a solution enriched with lanthanides and thorium. The thorium can be removed by forming thorium phosphate, a solid that settles out of solution. The lanthanides must then be removed from the excess phosphate, perhaps by precipitating them as lanthanide oxalates.

There are various ways in which the individual lanthanides can be separated from one another. Most methods are fractional in nature. They include precipitation as a solid, fractional decomposition of a solid by heat, extraction in solvents, and ion exchange.

Ion exchange is one of the quickest ways to separate individual lanthanides. It entails placing small beads of a hydrogen-exchange resin in a long, vertical column and passing the lanthanide-rich solution through the column. The lanthanides are exchanged for the hydrogen ions in the resin according to their size, with the larger ions being held more tenaciously than the smaller ions. Each step involves a gradual separation of each lanthanide element into fairly pure zones in the column. Then a chemical agent such as 0.1 percent citrate or EDTA solution over a restricted range of total hydrogen-ion concentration is passed through the column. The lanthanides are taken up selectively in the citrate or EDTA solution in the column and come out the bottom, where they may be collected in beakers at different times. The individual lanthanides that are collected in this manner are nearly 100 percent pure.

As technology continues to develop, scientists are discovering more and more uses for lanthanides. However, lanthanide-containing minerals are also becoming scarcer, necessitating the development of alternate sources. One potential solution is to extract and recycle lanthanides from electronic and other wastes.

Principal terms

ACID WATER: water with abundant hydrogen ions relative to hydroxide ions

ATOM: the smallest part of an element that can exist and retain the properties of that element

ATOMIC NUMBER: the number of protons in the nucleus of an atom

COMPOUND: a chemical combination of two or more elements in a definite ratio of atoms of each element

F-ELECTRON SHELL: an inner electron shell present in all lanthanides that can hold up to fourteen electrons

ION: a charged atom or molecule

NUCLEUS: the small, central portion of the atom that contains the positively charged protons and the neutral neutrons

OXIDATION: the removal of electrons from atoms to form positively charged ions

REDUCTION: the addition of electrons to atoms to form negatively charged ions

SOLUBILITY: how readily a compound dissolves in a liquid such as water

Bibliography

Atwood, David A., ed. The Rare Earth Elements: Fundamentals and Applications. Chichester: Wiley, 2012. Print.

Cotton, S. A., and F. A. Hart. The Heavy Transition Elements. London: Macmillan, 1975. Print. Includes a section on the lanthanide elements and one on the related actinide group.

Guangxian, Xu, and Xiao Jimei, eds. New Frontiers in Rare Earth Science and Applications. Orlando: Academic, 1985. Print. Contains articles on a variety of applications of the lanthanide elements, such as separation from ores, metallurgy, chemistry, spectroscopy, catalysis, medicine, toxicity, and applications. Many articles are suitable for someone with some general chemistry.

Koerth-Baker, Maggie. "4 Rare Earth Elements That Will Only Get More Important." Popular Mechanics. Hearst Communication, May 2012. Web. 2 Jan. 2014.

McCarthy, Gregory J., James J. Rhyne, and Herbert B. Silber, eds. The Rare Earths in Modern Science and Technology. 3 vols. New York: Plenum, 1977–82. Print. Describes the use of the lanthanides in hydrogen storage, chemistry (hydrides, catalysts, bioinorganic chemistry, solid-state chemistry), metallurgy, magnetism, optics, lasers, fluorescence, and spectroscopy. Suitable for someone with some general chemistry.

Michelsen, O. B. Analysis and Application of Rare-Earth Materials. Oslo: NATO Advanced Study Inst., 1973. Print. Describes analytical techniques for lanthanide analysis, including emission and flourescence spectroscopy, x-ray flourescence, isotope dilution, and neutron activation, and uses such as applications in optical devices, red phosphors for color television and lighting, ceramics, glasses, magnietic applications, luminescence, and geologic applications. Most articles can be read by someone with a general chemistry background.

Moeller, T. The Chemistry of the Lanthanides. New York: Reinhold, 1963. Print. Well written for the general reader with some chemistry background. Includes sections on the history, atomic structure, oxidation states, occurrence, and recovery of the lanthanides and a section on the related actinide elements.

Tabuchi, Hiroko. "Japan Recycles Minerals from Used Electronics." New York Times. New York Times, 4 Oct. 2010. Web. 2 Jan. 2014.

Topp, N. E. The Chemistry of the Rare-Earth Elements. Amsterdam: Elsevier, 1963. Print. Describes a variety of topics concerning the lanthanides with a minimum of chemical complexity. Includes such topics as the history of discovery, electronic structure, magnetic properties, extraction from minerals, separation techniques, lanthanide compounds and metals, analytical methods, and applications.

Trifonov, D. N. The Rare-Earth Elements. New York: Macmillan, 1963. Print. Has a long section on the history of the discovery and separation, why the family is peculiar, methods of separation, practical applications, and problems. Requires a minimal chemistry background.

United States. Environmental Protection Agcy. Rare Earth Elements: A Review of Production, Processing, Recycling, and Associated Environmental Issues. Washington: EPA, 2012. PDF file.

Acids and Bases

Chemical Formulas and Combinations

Solutes and Precipitates