The actinides (or actinoids) are the chemical elements with atomic numbers between 90 and 109 inclusively. (An atomic number indicates the number of protons in an atom.) Actinides occur between Groups 3 and 4 in Period 7 of the periodic table. All elements in this family are radioactive (that is, they spontaneously release subatomic particles or energy as their nuclei decay). Five actinides have been found in nature: thorium, protoactinium, uranium, neptunium, and plutonium. The other actinides have been produced artificially in nuclear reactors or particle accelerators (atom-smashers).
For many years, the list of chemical elements known to scientists ended with number 92, uranium. Scientists were uncertain as to whether elements heavier than uranium would ever be found. Then, in 1940, a remarkable discovery was made while University of California physicists Edwin McMillan (1907–1991) and Philip Abelson (1913– ) were studying nuclear fission. (Nuclear fission is the splitting of an atomic nucleus, a process that releases large amounts of energy. Atomic bombs and nuclear power plants operate on nuclear fission.) During their research, the duo found evidence for the existence of a new element with atomic number 94, two greater than that of uranium.
This new element was the first transuranium (heaver than uranium) element ever discovered. McMillan and Abelson named it neptunium, after the planet Neptune, just as uranium had been named after the planet Uranus. Later in the same year, McMillan and two other colleagues found a second transuranium element, which they named plutonium, after the planet Pluto.
At that point, the race was on to develop more synthetic transuranium elements, but the research process was not easy. The approach was to fire subatomic particles or small atoms, like those of helium, at a very large nucleus by means of a particle accelerator. If the smaller particle could be made to merge with the larger nucleus, a new atom would be produced. Over time, techniques became more and more sophisticated, and ever-heavier elements were created: americium (number 95) and curium (number 96) in 1944; berkelium (number 97) in 1949; californium (number 98) in 1950; einsteinium (number 99) and fermium (number 100) in 1952; mendelevium (number 101) in 1955; nobelium (number 102) in 1958; and lawrencium (number 103) in 1961.
Studies of the actinide elements are among the most ingenious in all of chemistry. In some cases, no more than one or two atoms of a new element have been produced. Yet scientists have been able to study those few atoms well enough to discover basic properties of the elements. These studies are made even more difficult because most actinide isotopes (atoms of a chemical element that are similar but not exactly alike) decay quickly, with half-lives of only a few days or a few minutes. (A half-life is the amount of time required for half of the atoms of a radioactive substance to disintegrate.)
With the discovery of lawrencium, the actinide family of elements is complete. Scientists have also found elements heavier than lawrencium, but these elements belong to the lanthanide family (or rare earth elements).
Uranium is a dull gray metallic element with a melting point of 1,135°C (2,075°F) and a boiling point of 4,134°C (7,473°F). It is relatively abundant in Earth's crust, ranking number 47 among the elements. Although perhaps not as well known, it is actually more abundant than more familiar elements such as tin, silver, mercury, and gold. Natural uranium consists of three isotopes of mass numbers 234 (0.005 percent), 235 (0.711 percent), and 238 (99.283 percent). All three isotopes are radioactive.
Properties and uses. By far the most important property of uranium is its radioactivity. Its most abundant isotope, uranium-238, decays by emitting an alpha particle with a half-life of 4.47 × 10 9 years. (Recall that the half-life of a radioactive element is the time it takes for one-half of a given sample to decay.) The half-life of uranium-238 is about equal to the age of Earth. That means that about one-half of all the uranium found on Earth at its moment of creation is still here. The other one-half has decayed to other elements.
Knowing the half life of uranium-238 (and many other radioactive isotopes) allows scientists to estimate the age of rocks. The amount of uranium-238 found in any particular rock is compared to the amount of daughter isotopes found with it. A daughter isotope is an isotope formed when some parent isotope, such as uranium-238, decays. The more daughter isotope present in a sample, the older the rock; the less daughter isotope, the younger the rock.
The second most abundant isotope of uranium, uranium-235, has the rare property of being fissionable, meaning that its atomic nuclei will break apart when bombarded by neutrons. The fission of a uranium-235 nucleus releases very large amounts of energy, additional neutrons, and two large fission products. The fission products are the atomic nuclei formed when a fissionable nucleus such as uranium-235 breaks apart.
The fission of uranium-235 nuclei has become extremely important in the manufacture of nuclear weapons and in the operation of nuclear power plants. In fact, these applications account for the primary applications of uranium in everyday life.
Thorium is a soft metal with a bright silvery luster when freshly cut. It has a melting point of about 1,700°C (3,100°F) and a boiling point of about 4,500°C (8,100°F). It is relatively soft, with a hardness about equal to that of lead. It is even more abundant than uranium, ranking number 39 in abundance among the elements in Earth's crust.
No more than a few hundred tons of thorium are produced annually. About one-half of this production goes to the manufacture of gas mantles, insulated chambers in which fuel is burned. The rest goes for use as nuclear fuel, in sunlamps (electric lamps that emit radiation; often used for tanning), in photoelectric cells (vacuum tubes in which electric current flows when light strikes the photosensitive—or light sensitive—cathode), and in the production of other alloys (a mixture of two or more metals or a metal and a nonmetal).
At one time, the actinides other than uranium were no more than scientific curiosities. They were fascinating topics of research for scientists but of little practical interest. That situation has now changed, and all of the actinides that can be prepared in large enough quantities have found some use or another. Plutonium, for example, is used in the manufacture of nuclear weapons and as the power source in nuclear power plants. On a smaller scale, it is also used as a power source in smaller devices such as the heart pacemaker. Californium is used in smoke detectors, curium is a power source in space vehicles, and americium is utilized in the treatment of cancer.