Actinides

Type of physical science: Chemistry

Field of study: Chemistry of the elements

Actinides are very heavy elements at the bottom of the periodic table, in a series that begins with actinium itself. All are radioactive, a property that dictates most of their common applications.

Overview

The actinides are a series of fourteen elements at what is currently the "end" of the periodic table of the elements. The series begins with element 89, actinium, and ends with element 102, nobelium. Element 103, lawrencium, is often considered a member of the series, and the next six elements or so, together with the actinides themselves, are often treated as a single group of "transuranium" elements (even though uranium, element 92, is the third member of the series). The table shows the actinide elements and the remaining transuranium elements, with physical and chemical information.

What unites the actinide elements proper is their electron configuration. The actinides follow the group I and II elements of period 7, francium (Fr) and radium (Ra), with electron configurations [Rn]7s¹[Rn]7s². The actinides fill in the inner 5f subshell, which is the next energy level above the 7s. As the 5f subshell can accommodate fourteen electrons, there are fourteen actinide elements. The table shows that this is a somewhat idealized explanation. The 6d and 5f subshells are so close in energy level that, in fact, the first three actinides add electrons in the 6d; only after that do they settle down and fill in the 5f subshell in orderly fashion. In this, they reflect the behavior of the lanthanides (elements 57 through 70, directly above the actinides in the periodic table), which fill in the 4f subshell, but begin by adding to the 5d subshell.

Lawrencium, with its 5f¹⁴6d¹ configuration, is properly the first member of a fourth d-transition series. The transuranium elements with generic names (based on their atomic numbers) continue that series.

The first four actinides are widely distributed in nature, and two of them (thorium and uranium) are more abundant than silver or mercury. Thorium was isolated from the ores thorite and thorianite and identified by Jons Jakob Berzelius in 1828. It is recovered commercially from the mineral monazite. Uranium was a suspected new element in the well-known ore pitchblende (from Bohemia, now in Czechoslovakia) in the eighteenth century, but was not finally isolated until 1841. Uranium is found in a number of other ores, as well. Actinium and protactinium occur in trace quantities in uranium ores. Although neptunium and plutonium in quantity are man-made, traces of these elements occur in association with particularly rich uranium deposits, where they are produced by the same nuclear reactions that are carried out more efficiently in the laboratory or in a nuclear reactor.

Neptunium, plutonium, and the remaining actinides are artificially produced by a variety of nuclear transformations. The first two elements are made by bombardment of uranium 238 with slow neutrons in enriched-uranium reactors with a fairly high neutron flux. The uranium-238 nucleus absorbs a neutron and emits a γ ray (a quantum of energy associated with a rearrangement of nucleons), becoming a uranium-239 atom. This decays by emission of β particles (electrons), which has the effect of raising the atomic number of the atom with each emission. The first product is neptunium 239, with a 2.33-day half-life. It decays to plutonium 239, a source of fission-produced nuclear power, with a half-life of 24,360 years. By extending nuclear γ- and β-decay reactions, isotopes of americium and curium can be produced.

Beyond these elements, other methods are used. Helium ions (α particles, consisting of two protons and two neutrons) are accelerated in a cyclotron and directed into the nuclei of americium 241 or curium 242, where the nuclei absorb the α particles and give off two neutrons to produce berkelium 243 and californium 244, respectively. For the remainder of the actinides, this type of synthesis can be used, but more direct methods, using heavier particles for bombardment, have been developed.

As the data in the table show, the actinide elements become less stable with respect to nuclear decay as their atomic numbers increase; that is, their half-lives become shorter, in general. All decay by spontaneous emission of α and β particles, descending in atomic number and mass through the nonactinide radioactive elements radium and radon and ending at one or another of the stable isotopes of element 82 (lead) or 83 (bismuth). Decay by fission is also possible, that is, by the element's nuclei splitting into pieces of approximately half the atomic number and mass, with the accompanying release of neutrons and enormous amounts of energy. Fission is initiated by neutrons and produces neutrons that can initiate other fissions; it is thus a chain reaction. Controlled, such a reaction is the source of power in nuclear generating stations; uncontrolled, it becomes the explosion of nuclear weapons like those tested at Alamogordo and released above Hiroshima and Nagasaki.

The chemistry of the actinides has been thoroughly investigated for the lower members, less so for the short-lived members at the end of the series. Although thorium and uranium, whose stable isotopes decay very slowly, have long been used in the laboratory and even in industry with no more precautions than one would take with any highly toxic reagent, the bulk of the actinides require remote handling to protect chemists who work with them from their radiation. Americium, to take only one example, is typically handled in a closed, shielded box with a 1-meter water wall between operator and sample.

The unoxidized actinide metals are known from actinium through americium. They show typical metallic properties of color and luster, workability, and so on, although some are so reactive that, like the group I and II metals, they quickly acquire an opaque coating through air oxidation. The table shows the oxidation states of the actinides. Like the lanthanides, they all show a +3 oxidation state, usually as the principal state. Like transition metal and lanthanide compounds, many of the actinide compounds are colored in both solid and solution phase.

Compounds investigated include oxides (oxides of uranium and plutonium are used as nuclear fuels); halides; compounds with other anions such as sulfate, nitrate, and oxalate; complexes with a great variety of ligands, inorganic and organic; and even some organometallics. For the transuranium elements beyond the actinides, progressively less chemistry is known. Although elements through element 110 have been produced by the heavy-particle bombardments described above, the combination of exceedingly short half-life and production in quantities ranging from a few thousand down to only a few atoms militates against leisurely chemical evaluation.

Applications

Nearly all the applications of the actinide elements depend on their radioactivity, not their chemistry. The few exceptions are shown by the longest-known members of the series, thorium and uranium. The principal use for thorium is in the Welsbach mantles used in portable gas lanterns. Thorium oxide, with small amounts of cerium oxide and other lanthanide compounds, is used because it incandesces in the gas flame with a pure white light of extraordinary intensity. Glasses made with thorium oxide have a high refractive index and low dispersion (tendency to produce a spectrum) and are used for lenses in cameras and scientific instruments. The oxide is also used in chemical manufacture, where it is a part of catalyst systems in production of nitric and sulfuric acids, and in petroleum cracking. Thorium metal is used as a coating for tungsten wires in electronic equipment because it is a good electron emitter.

It is also an important alloying element for magnesium.

Uranium has fewer commercial chemical applications than thorium. The oxide has been used in ceramic glazes and in making yellow "Vaseline" glass, in formulations that date back to the first century A.D. Uranyl nitrate is highly soluble in a number of organic solvents, a property that has been exploited in recovery and purification of uranium. The nitrate has also been used as a photographic toner. Uranyl acetate finds use in analytical chemistry, as it is one of the few reagents capable of precipitating sodium ions for quantitative measurement. Uranium metal has been used as a target in X-ray tubes for high-energy X rays.

Both thorium and uranium have been used to estimate the age of igneous rocks. As the table shows, both have half-lives in the billions of years, and measurement of the remaining thorium or uranium versus the lead isotope it decays to shows what portion of a half-life has passed since the element was incorporated into the rock. This allows dating of geological formations far into the Precambrian era. The natural radioactive decay of thorium and uranium is thought to account for the heat produced in the earth's interior.

Of the applications that make use of the radioactivity of the actinides, the most visible are nuclear power and nuclear weapons, though these are by no means the only applications. The nuclear reactor used for power generation uses uranium oxide in which the normal 0.7 percent uranium-235 level is increased to 3.3 percent (the remainder is uranium 238 with a trace of uranium 234). This is enriched uranium; the uranium 235 is added because it is the fissionable isotope that provides enough neutrons to keep a chain reaction going and to control the speed of that reaction by absorbing some of the neutrons with control rods of carbon or boron. The heat of the nuclear reaction is absorbed by water, and except for design features to eliminate the possibility of radioactive contamination, the remainder of the technology is standard steam-turbine electrical generation. As the uranium fuel "burns," some of the neutrons react with uranium 238 to produce plutonium 239 and other actinides. These come to produce more and more of the energy from the nuclear fuel, but eventually the fuel must be removed and stored in subcritical masses that cannot support the chain reaction, as plutonium 239 is fissionable and must not be allowed to build to too high a level. It can be recovered from spent fuel and redistributed into other reactors. Nuclear power is an important source of electricity in the industrialized nations. In the late twentieth century, most European countries generated about half their electricity in this way; for Belgium and France, the figure rose to about 70 percent.

Even the United States gained 20 percent of its power from nuclear sources; in absolute quantity, this was more than any other country in the world.

Nuclear weapons use the pure fissionable isotopes uranium 235 or plutonium 239.

When these are moderated by the presence of uranium 238 in a reactor, they can achieve the critical mass necessary to sustain a chain reaction, but not the supercritical mass necessary for explosion. When the isotopes are pure, however, it is possible to assemble enough fissioning nuclei that the neutron flux they produce can cause them all to fission in less than a microsecond.

The resulting release of energy is a nuclear explosion; moreover, the neutrons released at the same time are lethal to living creatures within several hundred meters, and the fission products are themselves radioactive and can contaminate a large area downwind of the explosion. The supercritical masses are not very large: about the size of a soccer ball for uranium, the material of the "Little Boy" bomb dropped at Hiroshima; and a softball for plutonium, used in the "Fat Man" bomb at Nagasaki. The technology of the bombs is concerned with getting two or more subcritical masses together very rapidly into the supercritical mass that explodes; the solution is some kind of chemical explosion that drives them together.

Of the other actinides, americium 241 has become a workhorse in the industrial world, and californium 252 has some important applications. Americium is used in thickness gauging in manufacture of foils, hot- and cold-rolled steel plate, flat glass, and a host of other products. The principle is simple: Measure how much radiation comes through the product being monitored.

Americium's advantage over other isotopes is that it emits α particles that can be used on very thin films, and a fairly penetrating γ ray that serves for thick materials. Americium is also used in home smoke detectors, where its α activity maintains a population of ions in a detector chamber that conducts a minute electric current. When the highly ionized particles produced by burning substances enter the chamber, they alter this current and set off the alarm, usually before smoke is dense enough to trigger optical devices.

Californium 252 is a strong neutron emitter, and can be used to estimate quantities of many elements by neutron activation analysis. In this procedure, a sample is exposed to a neutron source; nuclei capture neutrons and are made radioactive, and measurement of the energies of their decay products reveals what elements are present and in what quantities. The most useful application of this process is bore-hole logging, in which a californium source, with suitable detectors, is lowered into a hole drilled through a possible mineral deposit. The signals sent back by the activated nuclei can show the presence of valuable metals, or of water and oil layers.

Californium is also used as a start-up source in nuclear reactors, to calibrate the monitoring instruments before the fuel is actually installed, and as a scanner for fuel rods, to determine by neutron activation whether the rods contain the proper amount of fissionable material, evenly distributed.

Context

The history of the actinides and other transuranium elements is a twentieth century history; much of it, in fact, takes place in the second half of the twentieth century. The periodic table did not have a clear-cut place for actinides until the 1940's. Thorium was long thought to be a group IV element. Only when it was viewed with the other actinides could it be correctly cataloged.

The history of the actinides is also to a marked degree an American history. The early workers in the field were Europeans such as Enrico Fermi and Lise Meitner, who fled to the United States with the rise of Benito Mussolini and Adolf Hitler, and who gathered about them a number of remarkable American physicists and chemists; notable among them were Glenn Theodore Seaborg, Edwin Mattison McMillan, Ernest Orlando Lawrence, and J. Robert Oppenheimer. In work done at the University of Chicago, the Argonne National Laboratories, and the University of California at Berkeley, elements of the entire actinide series were produced between 1940 and 1961. By 1944, enough of them had accumulated that Seaborg was able to recognize them as a new series corresponding to the lanthanides and to rearrange the periodic table accordingly. For these theoretical advances, as well as for the synthetic nuclear reactions that produced the new elements, and the investigation of their chemistry, Seaborg and McMillan were awarded the 1951 Nobel Prize in Chemistry.

After a hiatus of almost a decade, elements 104 through 110 began to yield to investigation. The Berkeley group continued to be active, often with Albert Ghiorso as principal investigator; however, now other laboratories could claim prior or simultaneous discovery: the Gesellschaft fur Schwerionenforschung in Darmstadt, Germany, and the Soviet Union's Dubna Laboratory. It is clear from the spectra and the chemical properties of these elements that Seaborg's revision of the periodic table is correct: They are 6d transition elements, and if the ever-increasing problems of low yield and short half-life do not prove insurmountable, chemists can expect to see the last of them with element 112, and after that, six more elements to complete period 7 of the periodic table.

Much of the story of discovery of the actinides and their invaluable applications has been thrust into the background by the huge and chilling story of the atomic bomb, which was written at the beginning of this era of discovery. For half a century, human beings have lived with the knowledge that their world could stress human responsibility in wiping out the world.

This has given the actinides an unwarranted bad press with many people and obscured the fact that actinides, and radioactive elements in general, have far more benign applications than destructive ones.

Principal terms

ALPHA PARTICLE: a helium nucleus, consisting of two protons and two neutrons, thus with a charge of +2; given off by nuclei in radioactive decay

BETA PARTICLE: an electron, with a charge of -1, given off by a nucleus in radioactive decay

CHAIN REACTION: a reaction in which the products of one step in the reaction bring about the next step of the reaction, whose products initiate the first step again to produce an ongoing process; the fission of uranium 235 is one example

FISSION: a nuclear reaction in which a nucleus of a heavy element, struck by a neutron, breaks apart into two nuclei of approximately half the size of the original nucleus, with concomitant emission of more neutrons and much energy

GAMMA RAY: a quantum of energy emitted in certain nuclear reactions, sometimes inaccurately called a γ particle

ISOTOPES: atoms of the same element, with the same number of protons in their nuclei and hence the same atomic number, but with different numbers of neutrons in their nuclei and hence different mass numbers

LANTHANIDES: the group of fourteen 4f transition elements in period 6 of the periodic table, corresponding to the actinides and listed right above them, and with highly similar chemistry

PERIODIC TABLE OF THE ELEMENTS: an arrangement of the known elements in order of increasing atomic number, in which certain chemical and physical properties recur periodically and allow the elements to be organized in vertical groups of elements with similar properties

SUBSHELL: in the "shells" of electrons that surround a nucleus are smaller groupings of electrons called "subshells"; in the shells from 4 outward, these subshells are designated s, p, d, and f, and can hold two, six, ten, and fourteen electrons, respectively

Bibliography

Cotton, F. Albert, and Geoffrey Wilkinson. ADVANCED INORGANIC CHEMISTRY. New York: Interscience/Wiley, 1962. Old but not out of date, as much of the chemistry of the actinides was known by the time of publication. Assumes thorough knowledge of basic chemistry.

Greenwood, Norman N., and Alan Earnshaw. CHEMISTRY OF THE ELEMENTS. New York: Pergamon Press, 1984. Advanced, but includes details on the chemistry of the actinides and a certain amount on their applications. Though technical and somewhat difficult, this book is recommended because the standard college-level chemistry texts have little discussion about actinides beyond acknowledging their existence and place in the periodic table.

Kauffman, George B. "Beyond Uranium." CHEMICAL AND ENGINEERING NEWS 68 (November 19, 1990): 18-29. Discusses the researchers who discovered the actinide elements and the circumstances surrounding the discoveries. Illustrations.

Navratil, James D., Wallace W. Schulz, and Glenn T. Seaborg. "The Most Useful Actinide Element: Americium-241." JOURNAL OF CHEMICAL EDUCATION 67 (1990): 15-16. A short but complete discussion of how americium 241 is produced and isolated, what some of its compounds are, and in what applications it is important.

Plane, Robert A., and Ronald E. Hester. ELEMENTS OF INORGANIC CHEMISTRY. New York: Benjamin, 1965. A rather truncated discussion of actinides, but at a less advanced level than other texts.

Seaborg, Glenn T. "The Transuranium Elements." JOURNAL OF CHEMICAL EDUCATION 62 (1985): 463-467. Brief; more historical than technical, but authoritative.

Seaborg, Glenn T., et al. "Reflections on Nuclear Fission at Its Half Century." JOURNAL OF CHEMICAL EDUCATION 66 (1989): 362-393. Six articles in the May, 1989 issue under this collective title (one by Seaborg) discuss production methods, chemistry, theory, laboratory techniques, and other pertinent information.

Seaborg, Glenn T., and Walter D. Loveland. THE ELEMENTS BEYOND URANIUM. New York: Wiley, 1990. Leaves out the first four actinides except as they relate to the last ten, but is very thorough otherwise on production, chemistry, and applications. Somewhat technical, but manageable, because Seaborg's writing is a model of clarity.

Taylor, John J. "Improved and Safer Nuclear Power." SCIENCE 244 (April 21, 1989): 318-325. Discusses advances in reactor design. Readable. Contains diagrams of reactors.

Weast, Robert C., ed. HANDBOOK OF CHEMISTRY AND PHYSICS. 68th ed. Boca Raton, Fla.: C.R.C. Press, 1987. See particularly the article by C. R. Hammond, "The Elements," for information on sources, chemistry, and applications of each actinide; and the table that follows, "Physical Constants of Inorganic Compounds."

Actinides and other transuranium elements

The Effects of Nuclear Weapons

The Periodic Table and the Atomic Shell Model