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Atomic Number: 94
Atomic Symbol: Pu
Atomic Weight: 244
Melting Point: 1,184 F (640 C)
Boiling Point: 5,842 F (3,228 C)
Discovery: Plutonium is synthetically produced. It was discovered in 1940/41 by Glenn T. Seaborg, Edwin M. McMillan, J.W. Kennedy and A.C. Wahl at the University of California at Berkeley. Seaborg submitted a paper to the journal Physical Review in March 1941 documenting the discovery, but the paper was quickly withdrawn when it was found that an isotope of plutonium, Pu-239, could undergo nuclear fission, making it useful in developing an atomic bomb. The atomic bomb “Fat Man,” which was dropped on Nagasaki, Japan in 1945, had a plutonium core. The discovery of the new element was announced after World War II had ended.
Properties of plutonium
Plutonium is radioactive. Freshly prepared plutonium metal has a silvery bright color but takes on a dull gray, yellow, or olive green tarnish when oxidized in air. The metal quickly dissolves in concentrated mineral acids. A large piece of plutonium feels warm to the touch because of the energy given off by alpha decay; larger pieces can produce enough heat to boil water. At room temperature alpha-form plutonium (the most common form) is as hard and brittle as cast iron. It can be alloyed with other metals to form the room-temperature stabilized delta form, which is soft and ductile. Unlike most metals, plutonium is not a good conductor of heat or electricity. It has a low melting point and an unusually high boiling point. [See Periodic Table of the Elements]
Plutonium’s physiochemical complexities are unique among the elements because of its position in the periodic table. At plutonium’s location, 5f electrons are at the border between delocalized and localized behavior. It also sits near the place where the actinide series transitions from main d-block element chemistry to rare-earth like behavior because of actinide concentration. [Related: How Are the Elements Grouped?]
Plutonium metal normally has six allotropes or crystal structures; alpha (α), beta (β), gamma (γ), delta (δ), delta prime (δ') and epsilon (ε). It forms a seventh phase (zeta, ζ) under high temperature and a limited pressure range. These allotropes have very similar energy levels but very different densities and crystal structures. This makes plutonium extremely sensitive to changes in temperature, pressure, or chemistry, and allows for dramatic volume changes following phase transitions.
Plutonium can form alloys and intermediate compounds with most other metals, and compounds with a variety of other elements. Some alloys have superconductive abilities and others are used to make nuclear fuel pellets. Its compounds come in a variety of colors, depending on the oxidation state and how complex various ligands are. In aqueous solution there are five valance ionic states.
Plutonium-organic complexes are very important for separation, reprocessing, and purification.
Plutonium, along with all of the other transuranium elements, is a radiological hazard and must be handled with specialized equipment and precautions. Animal studies have found that a few milligrams of plutonium per kilogram of tissue are lethal.
Sources of plutonium
Though plutonium is synthetically produced, 1971 mass spectrometric measurements of plutonium isolated from Precambrian bastnasite confirmed that the isotope 244Pu is present in nature. 244Pu has a half-life of about 80 million years — just long enough for trace elements of primordial plutonium to still be found. Primordial 244Pu’s presence also means that plutonium is the heaviest primordial element in existence today.
Trace elements of plutonium are found in naturally occurring uranium ores. Here, it is formed in a way similar to neptunium: by irradiation of natural uranium with neutrons followed by beta decay.
Primarily, however, plutonium is a by-product of nuclear reactions in reactors where some of the neutrons released by fission convert 238U nuclei into plutonium.
Uses of plutonium
Plutonium’s most well-known use is as the explosive ingredient in nuclear weapons. Fission of a kilogram of 239Pu can produce an explosion as big as 21,000 tons of TNT.
There are attempts, however, to use spent nuclear fuel and excessive plutonium defense stockpiles for peaceful purposes. The PUREX chemical process extracts plutonium and uranium from spent nuclear fuel for reusing in nuclear power reactors. MOX fuel production reduces excessive plutonium stockpiles. Today, 239Pu is present in civilian nuclear power plants around the world.
Because of its high-heat production rate and long half-life of 88 years, 238Pu can be used as a heat source in radioisotope thermoelectric generators, which are used to power spacecraft and extraterrestrial rovers. It powered and heated instruments left on the moon by the Apollo astronauts, as well as weather satellites, interplanetary probes, the Cassini Saturn mission, and the Mars rovers. At one time, it was used to power artificial heart pacemakers and was studied as a method of providing supplemental heat to scuba divers. Mixed with beryllium, it is a convenient way to generate neutrons.
Isotopes of plutonium
Plutonium has 23 known isotopes. All are radioactive and range in mass from 228 to 247. Most of the isotopes have half-lives less than 7,000 years. Nine of the isotopes exhibit metastable states, though they have half-lives of less than one second. The longest living isotopes are 244Pu (half-life of 80.8 million years) 242Pu (half-life 373,300 years), and 239Pu (half-life of 24,110 years).
Isotopes with mass numbers lower than 244Pu decay primarily by spontaneous fission and alpha-emissions. As decay products, they form uranium and neptunium isotopes along with other daughter fission products. Isotopes with mass numbers higher than 244Pu decay by beta-emission and mostly form americium isotopes as daughter decay products. 244Pu is the parent isotope of the neptunium decay series.
238Pu and 239Pu are the most widely synthesized isotopes and the most useful. 239Pu, which is used in nuclear weapons and reactors, is synthesized using uranium and neutrons through beta decay with neptunium as an intermediate. 238Pu is synthesized by bombarding 238U with deuterons.