Discovered | Name | Characteristics | Uses

Plutonium is a member of the actinide series of elements, which stretches from actinium (Ac) to lawrencium (Lr).

Plutonium was discovered by Seaborg, A.C. Wahl, and J.W. Kennedy at the University of California in early 1941 as part of the war time research on the transuranium elements and their use in atomic weapons. Seaborg and his associates irradiated uranium oxide with deuterons (the isotope of hydrogen with 1 neutron) in a cyclotron. Over a period of months they extracted from the target traces of a new element, number 94. Continued bombardment of uranium oxide led eventually to the isolation of pure plutonium(IV) oxide, PuO2, in August 1942. This was the first time that anyone had actually seen a purely synthetic element.

The element was named by Seaborg to follow from the names of uranium and neptunium. Pluto is the next planet in the solar system beyond uranus and neptune, so it was logical that the new element should be plutonium.

Unlike uranium and neptunium, which precede plutonium in the actinide series, Pu is a purely synthetic element. There are no traces of the element naturally occurring on the earth.

The melting point of Pu metal is relatively low for a metal (640°C). Further, at its melting point the density of liquid plutonium is 2.4% greater than that of the solid allotrope at that temperature. Consequently, the solid will float on the liquid, a property shared by only a very few other substances, most notably water.

The properties of Pu metal are surprisingly different from those of uranium. In addition, considering the usual properties of a metal - high thermal and electrical conductivity, good malleability - plutonium is a poor metal. Its thermal conductivity is only about 0.1 that of Ag and only ten times greater than fire brick, a thermal insulator. One explanation for the poorly metallic properties of Pu is the distribution of 5f electrons. (The element has six 5f electrons.) The actinide elements can be divided into two groups, those with relatively delocalized f electrons - which leads to metallic properties - and those in which the f electrons are more localized - which leads to less metallic properties. Plutonium is on the dividing line of these two types of 5f electron behaviors and so has unique properties.

Plutonium metal is highly reactive, combining with oxygen, CO, N2, H2, and other gaseous compounds. It is such a powerful reducing agent that the crucibles in which Pu chemistry is done must be extremely stable oxides, carbides, or nitrides of other elements.

The metal can exist in six allotropic forms at normal pressure and in a seventh at higher pressures. Many other elements have allotropic forms, but only Pu has so many.

Plutonium forms many different compounds in oxidation states of +3 through +7. Oxygen compounds are numerous as are halides. Yellow-brown plutonium(IV) oxide, PuO2, has been studied extensively because it is potentially useful as a nuclear fuel. Plutonium(VI) fluoride, PuF6, is also very interesting. Unlike UF6, which is a volatile solid, PuF6 is a liquid. It is also a powerful fluorinating agent, able to convert SF4 to SF6, for example, and to react with xenon.

The solution chemistry of plutonium is characterized by five oxidation states ranging from +3 to +7 and by great complexity. Plutonium(III) ion is stable to water and air, but it can be oxidized by mild oxidants. Plutonium(IV) is best considered as the oxycation [PuO2]+, which is stable only in very concentrated acid since extensive hydrolysis occurs even at a pH as low as 1.

Plutonium has 15 isotopes, all radioactive. 244Pu is the longest lived, with a half-life of 82,000,000 years.

Plutonium and its compounds are intensely radioactive. Therefore, it has been suggested for use in thermoelectric power devices in satellites and in neutron activation analysis.

For more information see G.R. Choppin and B.E. Stout, Chem Brit. December, 1992, page 1126.