The Actinides

 

The actinides, elements 90-103, follow actinium on the periodic table.  They have electron configurations of 5f^x 6d¹ 7s².  With the exceptions of actinium, thorium, and uranium, the actinides are not found naturally, and are instead synthetically produced by neutron bombardment or in particle accelerators.

 

Thorium (Th, Z=90)

Thorium is a soft, silvery, radioactive metal.  It is named for Thor, the Norse god of thunder.  It is found in the Earth's crust at a concentration of 12 ppm, making it the 37th most abundant element, and the most abundant of the actinides.  It is found in the ores thorite [ThSiO₄], uranothorite [(U,Th)SiO₄], and thorianite [ThO₂].  It is also found in monazite (see introduction for the lanthanides section) as thorium oxide, ThO₂, at concentrations up to 12%.

Thorium reacts slowly in air and water.  The oxide forms a protective coating over the metal, making it stable at high temperatures.  In its compounds, it is found in the +4 oxidation state.  All of the isotopes of thorium are radioactive; the most stable isotope is thorium-232, with a half-life of 14,000,000,000 years; this isotope comprises almost 100% of the world's naturally occurring thorium.  Because thorium is a weak alpha emitter, it is relatively safe to use in some commercial applications.  Thorium is used as an alloying agent for some metals, such as magnesium, to improve their high-temperature strength.  It is also used in elecronic photosensors to measure ultraviolet light.

Thorium(IV) oxide, ThO₂, used to be used in mantles in portable gas lights.  It is an ingredient in some high-quality lenses, since it forms glasses with a high refractive index and low dispersion.  It is also used in high-temperature laboratory crucibles, since it has a very high melting point of 3300°C.  It is also used as a catalyst for the conversion of ammonia to nitric acid, in petroleum cracking, and in the production of sulfuric acid.

Thorium-232 can be converted into thorium-233 by bombardment with neutrons, thereupon decaying into protactinium-233 and then into uranium-233.  Uranium-233 undergoes nuclear fission in a chain reaction, and this cycle has potential to be used in nuclear fusion plants.  Because thorium is more abundant than uranium, this may hold some promise in later generations of nuclear power plants.

 

Protactinium (Pa, Z=91)

Protactinium is a silvery-white, radioactive metal.  Its name is derived from the Greek word proto and actinium, meaning "parent of actinium," because it undergoes radioactive decay to produce actinium.  It is found in the Earth's crust in trace amounts, and is among the ten least abundant elements.  It is found in uranium ores such as pitchblende, at very low concentrations.

Protactinium is extremely radioactive.  It oxidizes slowly in air.  In its compounds, it is found in the +4 and +5 oxidation states.  There are two naturally occurring isotopes of protactinium:  protactinium-234, with a a half-life of 6 hours and 42 minutes, and protactinium-231, with a half-life of 32,500 years.  Because it is highly radioactive and toxic, there are no commercial applications for protactinium.

 

Uranium (U, Z=92)

Uranium is a lustrous, silvery-white, hard, dense, malleable, radioactive metal.  It is named for the planet Uranus, which had been discovered a few years before the discovery of the element in 1789.  (The planet Uranus had in turn been named for the Greek god of the sky.)  It is found in the Earth's crust at a concentration of 2 ppm, making it the 48th most abundant element.  It is found in the ores pitchblende [mostly uranium(IV) oxide, UO₂], uraninite [UO₂, UO₃, and traces of other minerals], autunite [Ca(UO₂)₂(PO₄)₂·(10-12)H₂O], carnotite [K₂(UO₂)₂(VO₄)₂·3H₂O], samarskite [(Y,Fe,U)(Nb,Ta)₅O₄, and traces of other rare earth elements], torbernite [Cu(UO₂)₂(PO₄)₂·(8-12)H₂O], betafite [(Ca,U)₂(Ti,Nb,Ta)₂O₆(OH)], uranophane [Ca(UO₂)₂(SiO₃)₂(OH)₂·5H₂O], and coffinite [U(SiO₄)_1-x(OH)_4x].  Lignite coal and monazite ore also contains trace amounts of uranium.

Uranium tarnishes in air to produce an oxide coating.  In its compounds, uranium is usually found in the +4 or +6 oxidation states.  In many compounds, uranium occurs as the uranyl ion, UO₂²⁺.

One of the most important properties of uranium was not discovered until 1896, when Henri Becquerel discovered that uranium is radioactive.  In one of the classic examples of "accidental" discovery in science, he placed a sample of uranium salts next to a photographic plate, intending to examine the phosphorescence of uranium when exposed to sunlight.  However, he discovered that the plates were exposed, even though the sample had remained in the dark, and realized that the uranium was spontaneously emitting a type of radiation that was not connected to any external stimulation.  He shared the Nobel Prize in Physics for the discovery of radioactivity with Pierre and Marie Curie in 1903.

There are three major naturally occurring isotopes of uranium (there are quite a few other isotopes, but their half-lives are relatively short, and they are not found in any significant amounts in natural sources).  The major isotope is uranium-238, which has an abundance of 99.275%; it has a half-life of 4.46´10⁹ years.  It emits alpha particles and gamma rays, and is non-fissionable, absorbing neutrons instead of splitting.  Uranium-235, which has an abundance of 0.720%, has a half-life of 7.04´10⁸ years.  It also emits alpha particles and gamma rays, and is fissionable, splitting into lighter atoms when struck by a neutron.  Uranium-234, which has a half-life of 245,000 years, is found at very low concentrations, about 0.005%, and is produced as a part of the decay sequence of uranium-235.  Uranium-235 eventually decays into lead-206, and uranium-238 into lead-207.  The radioactive decay of uranium isotopes contributes a great deal to the internal heat of the Earth.  The relative amounts of uranium and lead in rocks can be used in radioactive dating techniques.

When neutrons are fired at uranium-235, the extremely unstable uranium-236 isotope is produced, which quickly splits into two smaller nuclei (such as barium and krypton), releasing some of the nuclear binding energy and more neutrons.  This process is known as nuclear fission.  These neutrons can collide with other uranium-235 isotopes, causing them to split and release energy and even more neutrons in a chain reaction.  In atomic power plants, fuel rods contain uranium or uranium oxide that is enriched in up to 2 to 3% uranium-235.  Moderators such as beryllium, graphite, water, or heavy water (D₂O) slow down the neutrons that are released in fission reactions, producing "thermal neutrons" that can be captured by uranium atoms, rather than simply bouncing off.  (In "light water" reactors, the moderator is ordinary water — i.e., water containing the hydrogen-1 isotope; these reactors require enriched uranium.  In "heavy water" reactors, the moderator is D₂O — i.e., water containing deuterium, the hydrogen-2 isotope; these reactors can use natural, unenriched uranium.)  The rate of the nuclear reactions are controlled with control rods, which contain elements that are capable of absorbing neutrons without undergoing fission, such as silver, indium, cadmium, boron, cobalt, hafnium, gadolinium, and europium.  Raising the control rods out of the reactor allows the reaction to speed up, while lowering them into the reactor slows the reaction down and prevents a runaway chain reaction from occurring.

In atomic bombs, the reaction goes out of control, causing a tremendous explosion which releases tremendous amounts of energy and radiation.  The "Little Boy" atomic bomb that was exploded in Hiroshima, Japan on August 6, 1945 was a uranium bomb which had the destructive potential of 12,500 tons of TNT, and killed over 75,000 people.  In this bomb, two pieces of sub-critical uranium were brought together quickly by explosives, making a critical mass of uranium in which a runaway chain reaction could occur.

One technique for enriching uranium with the fissionable uranium-235 isotope is by converting it into the form of volatile uranium hexafluoride, UF₆, which sublimes into the gas phase at 56.5°C.  The gas is then diffused through a series of permeable membranes:  since ²³⁵UF₆ is 3 atomic mass units lighter than ²³⁸UF₆, it diffuses 1.0043 times faster through the membranes, resulting in a final product which is enriched in uranium-235 relative to the starting material.  (Uranium hexafluoride is extremely corrosive:  during the Manhattan Project to build the first atomic bomb, Teflon was used to make gastight fittings for the valves and seals used in the gas diffusion equipment.)  Enriched uranium can also be prepared in a high-speed gas centrifuge, or by using a laser to dissociate the U—F bonds in UF₆, causing uranium-235 to deposit out.

Uranium-238 does not undergo fission, but when bombarded with neutrons, uranium-238 absorbs neutrons to produce plutonium-239.  Absorption of neutrons by uranium-235 produce plutonium-238.

Uranium-233 (half-life of 159,000 years) can be produced by neutron bombardment of thorium-232.  This uranium isotope also undergoes nuclear fission, and has the potential to be used in nuclear fusion plants.  Because thorium is more abundant than uranium, this may hold some promise in later generations of nuclear power plants.

Depleted uranium consists of uranium taken from spent fuel rods and alloyed with small percentages of other elements.  It contains mostly U-238, and less than 0.7% U-235 — usually about 0.2 to 0.4%.  It is extremely dense, and is used to make armor-piercing ammunition, and ballast for ships and airplanes.

Uranium glass is a yellow or yellow-green glass which is colored by uranium oxide.  Under ultraviolet light, uranium glass fluoresces with a bright green color.  By including other minerals, an opaque, yellow or white ceramic called "vaseline glass" can be produced (so called because its appearance is similar to petroleum jelly).  Small amounts of uranium is also found in the red-colored pieces in a line of dinnerware called Fiestaware; the so-called "Red Fiesta" coloring contains uranium oxide in the glaze, which produces a vivid reddish-orange color (see here for an example).

Uranium entering the body becomes concentrated in the bones because uranium forms complexes with phosphate ions.

 

Neptunium (Np, Z=93)

Neptunium is a silver, radioactive, artificially produced element.  It is named for the planet Neptune, since Neptune follows Uranus in the solar system.  It is found in uranium ores at very low concentrations, but the commercial source for neptunium is from spent uranium fuel rods from nuclear reactors.

The two longest-lived isotopes of neptunium are neptunium-237, which has a half-life of 2,140,000 years, and neptunium-236, with a half-life of 155,000 years.  All of the other isotopes have half-lives that range from a few minutes to a little over a year.

Neptunium is the first of the transuranium elements, which have higher atomic numbers than uranium.  With very few exceptions, the transuranium elements are too rare, and too dangerous, to find much commercial use.

Neptunium was first prepared in 1940 by Edwin M. McMillan and Philip H. Abelson at the University of California in Berkeley, California by bombarding uranium-238 with neutrons, producing uranium-239, which underwent beta decay to produce neptunium-239.  Uranium-235 can also be converted to neptunium by neutron bombardment, absorbing two neutrons to produce uranium-237, which then undergoes beta decay to produce neptunium-237.  Uranium-238 can also absorb one neutron, emit two more neutrons and become uranium-237, which then undergoes beta decay to produce neptunium-237.

 

Plutonium (Pu, Z=94)

Plutonium is a silver, radioactive, artificially produced element.  It is named for the planet (well, ex-planet) of Pluto, since Pluto follows Neptune (usually) in the solar system.  Its discoverer, Glenn T. Seaborg chose the symbol "Pu" for the element, rather than "Pl" "partly to avoid confusion with platinum, Pt, but also 'facetiously,' he says, 'to create attention' — P.U. the old slang for putrid, something that raises a stink."*  It is found in small quantities in uranium ores, produced from uranium-238 isotopes which capture neutrons emitted by uranium-235; it is one of the ten least abundant elements found in the Earth.

Plutonium was first synthesized by Glenn T. Seaborg, Arthur C. Wahl, and Joseph W. Kennedy at the University of California in Berkeley, California in 1940, although its existence was not reported publicly until 1946 because of the security restrictions surrounding nuclear research and the Manhattan Project to build the first atomic bombs.  Uranium-238 was bombarded with neutrons to produce uranium-239, which then underwent beta decay to produce neptunium-239, which also underwent beta decay to produce plutonium-239.

The longest-lived isotope of plutonium is plutonium-244, which has a half-life of 82,000,000 years; plutonium-242 and -239 also have fairly long half-lives, of 380,000 years and 24,000 years.  The most commonly used isotopes are plutonium-238 (half-life of 87.7 years) and plutoium-239.

Plutonium-238 is not fissionable, and emits alpha particles without also emitting gamma rays, making it a great deal safer to handle.  It is used primarily as a long-lived power source in pacemakers, spacecraft and satellites, and deep-sea diving suits.

Plutonium-239 is fissionable, and is used in atomic weapons.  The critical mass for plutonium-239 is about 16 kg, but this can be reduced by surrounding the plutonium with a shell of beryllium, which reflects neutrons back towards the plutonium, accelerating the fusion process.  One kilogram of plutonium-239 has the explosive equivalent of 20,000 tons of TNT.

Plutonium-240 (half-life of 6,500 years) is a contaminant in weapons-grade plutonium; since this isotope emits neutrons when it undergoes spontaneous fission, plutonium cannot be used in a gun-type device as uranium-235 can (see entry for uranium), because the device would be blown apart before a critical mass of plutonium-239 can undergo much fission.  This requires plutonium-based weapons to use a more complicated implosion design.

Plutonium was a component of the first atomic bomb to be detonated, at the Trinity test site near Alamagordo, New Mexico on July 16, 1945.  In this device, a shell of plutonium was imploded by conventional explosives, which compressed the plutonium together to form a critical mass; the explosion was initiated by a shower of neutrons emitted by a polonium source.  The first atomic bomb used in warfare was dropped on Hiroshima, Japan, on August 6, 1945, but this one used uranium only.  The "Fat Man" atomic bomb that was exploded in Nagasaki, Japan on August 9, 1945 was also a plutonium bomb which had the destructive potential of approximately 21,000 tons of TNT, and killed over 70,000 people. 

Plutonium bombs are also used to initiate the explosion of hydrogen bombs, which are powered by nuclear fusion instead of fission (see entry for Hydrogen on the Group 1A page).

 

Americium (Am, Z=95)

Americium is a silvery, synthetic, radioactive metal, produced by the neutron bombardment of plutonium.  It was first made by Glenn T. Seaborg, Ralph A. James, Leon O. Morgan, and Albert Ghiorso at the University of Chicago in 1944.  It was named for the American continent because of its chemical similarities to europium (the element immediately above it in the lanthanide series), which named for Europe.  The longest-lived isotope, americium-243, has a half-life of 7370 years; americium-241 has a half-life of 432 years.

Americium-241 is used in gas and smoke detectors, in the form microgram quantities of americium oxide, AmO₂.  The alpha particles that the isotope emits ionizes the air in the space between the electrodes in the detector, causing an electrical current to flow between the electrodes.  When smoke enters the detector, the current flow is interrupted or reduced because the ions are absorbed by the smoke particles, triggering the alarm to sound.  The alpha particles emitted by the americium do not pose any health hazard, because they are easily blocked by the metal and plastic in the detector housing, and pick up electrons to become harmless helium atoms.

 

Curium (Cm, Z=96)

Curium is a silvery, synthetic, radioactive metal, produced by bombarding plutonium-239 with alpha particles.  It was first made by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso at the University of California, Berkeley, in 1944.  It was named in honor of Marie and Pierre Curie, the co-discovers of radioactivity.  The longest-lived isotope, curium-247, has a half-life of 16,000,000 years.

 

Berkelium (Bk, Z=97)

Berkelium is a silvery, synthetic, radioactive metal, produced by bombarding americium-241 with alpha particles.  It was first made by Stanley G. Thompson, Albert Ghiorso, and Glenn T. Seaborg at the University of California, Berkeley, in 1949.  It was named in honor of Berkeley, California, where it was first produced.  The longest-lived isotope, berkelium-247, has a half-life of 1400 years.

 

Californium (Cf, Z=98)

Californium is a synthetic, radioactive metal, produced by bombarding curium-242 with alpha particles.  It was first made by Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiorso, and Glenn T. Seaborg at the University of California, Berkeley, in 1950.  It was named in honor of the University of (and the state of) California.  The longest-lived isotope, californium-251, has a half-life of 898 years.

Californium is a powerful neutron emitter, and is used as a neutron source in moisture gauges, sensors on oil wells, and neutron activation analysis for measuring trace amounts of elements.  It is also used in the treatment of some cervical cancers.

 

Einsteinium (Es, Z=99)

Einsteinium is a synthetic, radioactive metal, produced by bombarding plutonium-239 with neutrons for several months; a complex series of alternating decay and capture processes eventually results in einsteinium-253.  It was first isolated in the residue of the "Mike" hydrogen bomb test on Elugelab Island in the Enewetak atoll of the Marshall Islands on November 1, 1952; around 200 atoms of einsteinium were identified in the tons of radioactive debris that were processed at the University of California and the Los Alamos National Laboratory by Gregory R. Choppin, Stanley G. Thompson, Albert Ghiorso, and Bernard G. Harvey.  The discovery was not announced publicly until 1955 because of security restrictions surrounding the bomb test.  It was named in honor of Albert Einstein.  The longest-lived isotope, einsteinium-252, has a half-life of 472 days.

 

Fermium (Fm, Z=100)

Fermium is a synthetic, radioactive metal, produced by bombarding plutonium-239 with neutrons for several months; a complex series of alternating decay and capture processes eventually results in fermium-253.  It was first isolated in the residue of the "Mike" hydrogen bomb test on Elugelab Island in the Enewetak atoll of the Marshall Islands on November 1, 1952; around 200 atoms of fermium were identified in the tons of radioactive debris that were processed at the University of California and the Los Alamos National Laboratory by Gregory R. Choppin, Stanley G. Thompson, Albert Ghiorso, and Bernard G. Harvey.  The discovery was not announced publicly until 1955 because of security restrictions surrounding the bomb test.  It was named in honor of Enrico Fermi.  The longest-lived isotope, fermium-257, has a half-life of 100 days.

Fermium is difficult to produce, and has only been made in less-than-picogram quantities, because fermium-257 absorbs neutrons easily, and when fermium is synthesized, goes on to form fermium-258, which has a half-life of 0.37 milliseconds.

 

Mendelevium (Md, Z=101)

Mendelevium is a synthetic, radioactive metal, produced by bombarding einsteinium-253 with alpha particles.  It was first made by Albert Ghiorso, Bernard G. Harvey, Greogory R. Chopin, Stanley G. Thompson, and Glenn T. Seaborg at the University of California, Berkeley, in 1955.  It was named in honor of Dimitri Mendelev, the deviser of the periodic table of the elements.  The longest-lived isotope, mendelevium-258, has a half-life of 51.5 days.

 

Nobelium (No, Z=102)

Nobelium is a synthetic, radioactive metal, produced by bombarding californium-249 with carbon-12 ions.  It was first made by by Albert Ghiorso, Torbjorn Sikkeland, John R. Walton, and Glenn T. Seaborg in 1958.  It was named in honor of Alfred Nobel, the inventor of dynamite and founder of the Nobel Prize.  The longest-lived isotope, nobelium-259, has a half-life of 58 minutes.

 

Lawrencium (Lr, Z=103)

Lawrencium is a synthetic, radioactive metal, produced by bombarding californium-252 with boron-10 ions.  It was first made by by Albert Ghiorso, Torbjorn Sikkeland, Almon Larsh, and Robert M. Latimer in 1961.  It was named in honor of Ernest O. Lawrence, inventor of the cyclotron, one of the earliest particle accelerators.  The longest-lived isotope, lawrencium-262, has a half-life of 216 minutes.

 

 

References

John Emsley, The Elements, 3rd edition.  Oxford:  Clarendon Press, 1998.

John Emsley, Nature's Building Blocks:  An Z-Z Guide to the Elements.  Oxford:  Oxford University Press, 2001.

David L. Heiserman, Exploring Chemical Elements and their Compounds.  New York:  TAB Books, 1992.

* Richard Rhodes, The Making of the Atomic Bomb.  New York:  Simon & Schuster, 1986, p. 414.