TRANSURANIUM ELEMENTS, chemical elements of atomic number greater than that of uranium (at. no. 92) in the periodic table (see Periodic Law). Of the known transuranium elements (23 as of 1999) only two-neptunium and plutonium-exist at all in nature; the others have been synthesized through nuclear reactions involving bombarding the atoms of one element with neutrons or fast-moving charged particles. These elements consist of more than 100 radioactive isotopes (see Isotope), which are characterized by various degrees of radioactive instability (see Radioactivity). These radioisotopes are produced artificially by bombarding heavy atoms either with neutrons produced in nuclear reactors or in specially designed nuclear explosives, or with charged particles accelerated to high energy in such devices as cyclotrons or linear accelerators. The first 11 transuranium elements, together with actinium, thorium, protactinium, and uranium, constitute the actinide elements chemically analogous to the Rare Earth Elements (q.v.001000228015000215). They are, in order, Neptunium, Plutonium, Americium, Curium, Berkelium, Californium, Einsteinium, Fermium, Mendelevium, Nobelium, and Lawrencium. Elements with atomic numbers transcending that of lawrencium are called transactinides. They are, in order of atomic weight, rutherfordium, dubnium, seaborgium, bohrium, hassium, meitnerium, and the yet unnamed elements-110, 111, 112, 114, 116, and 118. Also predicted, the elements 113, 115, and 117 have yet to be discovered.

Beginning with neptunium (at.no. 93), the first to be discovered, in 1940, and ending with lawrencium (at.no. 103), in 1961, all transuranium elements in the actinide series were produced by American scientists.

Other Discoveries

Between 1964 and 1984, scientists in the U.S., Europe, and the Soviet Union announced the definite or tentative production of six further transuranium elements beyond lawrencium in the periodic table, and hence beyond the actinide series. The first of these, element 104-rutherfordium (Rf) in the periodic table of elements-was reportedly produced in a heavy ion cyclotron at the Joint Institute for Nuclear Research at Dubna, near Moscow, in 1964, by irradiating a plutonium target with neon ions. A team at Lawrence Berkeley Laboratory led by the American scientist Albert Ghiorso (1915-    ) could not reproduce these results, but instead produced element 104 by bombarding californium with carbon atoms in 1969.

Element 105 (called hahnium from 1970 to 1997, when it was renamed dubnium) was also produced at Dubna in 1968 by bombarding americium with neon ions; Ghiorso's team achieved a similar result in 1970 by bombarding californium with nitrogen ions. In 1974 the Dubna group produced element 106-seaborgium (Sg) in the periodic table of elements-by bombarding lead isotopes with a beam of chromium; the American team produced it that same year by using californium and oxygen.

The production of element 107 was announced in 1977 by the Dubna research team, using a bismuth target and a beam of chromium, but was not confirmed elsewhere. Elements 107, 108, and 109-known as bohrium, hassium, and meitnerium, respectively-were synthesized by a team of researchers using the Universal Linear Accelerator (UNILAC) at the Institute for Heavy Ion Research in Darmstadt, Germany, beginning in 1981.

An intense 2-decade long competition within the scientific community surrounding the names of the elements with atomic numbers of 104 to 109 was finalized by the International Union of Pure and Applied Chemistry (IUPAC) in 1997. Based on the recommendation of its international naming commission, the names accepted were: rutherfordium (104); dubnium (105); seaborgium (106); borhium (107); hassium (108); and meitnerium (109).

Between 1994 and 1999, scientists from Germany, the U.S., and Russia reported six more transuranium elements. At Darmstadt, element 110 was created by bombarding lead with nickel, and element 111 by bombarding bismuth with nickel in 1994; two years later, element 112 (with 165 neutrons) was created by bombarding lead with zinc. An element 112 with 173 neutrons was obtained in 1998 by researchers at Dubna during a decay chain-reaction that resulted in the production of the element 114 and a series of its isotopes. Of those, the element 114 with 173 neutrons exhibited higher stability and a longer lifetime. The elements 116 and 118 were obtained by researchers at Lawrence Berkeley National Laboratory in 1999.

Heavier Transuranium Elements

The production of elements 114, 116, and 118 confirmed theoretical studies advanced as early as 1966, suggesting that such superheavy elements have comparatively stable nuclear arrangements. It has also been proven that production of such heavy nuclei necessitate accelerating much heavier ions than have been accelerated to date.

Production and Uses

The radioactive decay rates of the transuranium elements tend to increase with increasing atomic number; the very heavy transuranium nuclei, such as californium, tend to fission spontaneously. As a result, it is extremely difficult to manufacture large quantities of the elements heavier than plutonium. This problem is being attacked by bombarding uranium and plutonium with very intense streams of neutrons in reactors such as the High Flux Isotope Reactor at Oak Ridge National Laboratory in Tennessee. In the mid-1970s this reactor was producing several milligrams per year of berkelium, californium, and einsteinium, and small amounts of fermium. In addition, nuclear explosions, which release very high neutron fluxes, can be designed specifically to encourage the instantaneous production of the heavy elements einsteinium and fermium. Once sufficient quantities of the heavy elements are available, it should be possible to use isotopes such as plutonium-238 and curium-244 as extremely compact and dependable, although somewhat expensive, sources of power, with the radioactive-decay heat converted directly to electricity by thermoelectric devices. Other transuranium isotopes such as americium-241 and californium-252 have medical and industrial uses.        G.T.S.

See Atom And Atomic Theory; Elements, Chemical; Particle Accelerators.