Transuranium Elements

I

Introduction

Transuranium Elements, chemical elements of atomic number greater than 92, the atomic number of uranium in the periodic table (see Periodic Law). More than 20 such elements have been identified. These elements consist of more than 100 radioactive isotopes (see Isotope), which are characterized by radioactive instability (see Radioactivity). These radioisotopes are produced artificially by bombarding heavy atoms either with neutrons produced in nuclear reactors or with charged particles accelerated to high energy in particle accelerators. The first 10 transuranium elements, together with actinium, thorium, protactinium, and uranium, constitute the actinide elements, which are chemically analogous to the rare earth elements (see Actinide Series; Lanthanide Series). They are, in order, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, and nobelium.

II

Later Discoveries

From 1964 to 1984, scientists in the United States, Europe, and the Soviet Union announced the production of six further transuranium elements beyond nobelium in the periodic table, and hence beyond the actinide series. The first of these, element 104, rutherfordium (Rf), was reportedly produced at the Joint Institute for Nuclear Research in Dubna, Russia, in 1964, by irradiating a plutonium target with neon ions. A team led by American scientist Albert Ghiorso at Lawrence Berkeley National Laboratory in Berkeley, California, could not reproduce these results, but instead produced rutherfordium by bombarding californium with carbon atoms in 1969.

Element 105, dubnium (Db), was produced at Dubna in 1967 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), by bombarding lead isotopes with a beam of chromium, and the American team produced it that same year by using californium and oxygen.

Element 107, bohrium (Bh), was produced in 1977 by the Dubna research team, using a bismuth target and a beam of chromium. Element 108, hassium (Hs), and element 109, meitnerium (Mt), were synthesized in 1984 and 1982, respectively, by a team of physicists at the Institute for Heavy Ion Research in Darmstadt, Germany. In 1994 physicists at the Darmstadt laboratory produced two more elements: element 110, darmstadtium (Ds), by bombarding lead with nickel atoms; and element 111, roentgenium (Rg), by bombarding bismuth with nickel atoms. The discoveries of elements 112 and beyond have not yet been independently confirmed. In 1996 the production of element 112, ununbium (Uub), was announced by researchers at Darmstadt who produced the element by bombarding lead with atoms of zinc.

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Physicists at Dubna produced element 114, ununquadium (Uuq), in 1999 by shooting a beam of calcium atoms at a plutonium target. The ununquadium atoms they created lasted for about 30 seconds. Most of the heaviest transuranium elements have half-lives of only a fraction of a second. The unusual length of ununquadium’s half-life lends credence to theories predicting the existence of islands of stability containing superheavy artificial elements with relatively long half-lives. Dubna physicists produced element 116, ununhexium (Uuh), in 2001 by smashing calcium atoms in atoms of curium. The atoms of element 116 they created, which lie on the edge of the island of stability at 114, lasted only a fraction of a second.

In 2004 physicists at the Lawrence Livermore National Laboratory in the United States and at the Joint Institute for Nuclear Research in Dubna together announced the discovery of elements 115 and 113. Scientists smashed calcium atoms into an americium target to make element 115, which quickly decayed into element 113. Element 115 has the temporary name ununpentium (Uup) and element 113 the temporary name ununtrium (Uut).

In 2006 another joint team of U.S. and Russian researchers from the Lawrence Livermore National Laboratory and the Joint Institute for Nuclear Research reported detecting element 118, based on experiments conducted with the cyclotron at Dubna. Three atoms of element 118 were produced by bombarding a californium target with calcium ions. The new element had a half-life of only about a millisecond and decayed to element 116 and then to element 114 by releasing alpha particles (two protons and two neutrons bound together). Theory predicts that element 118—temporarily called ununoctium (Uuo)—should be a noble gas that would fit below radon on the periodic table of elements. A claim made by other researchers in 1999 that element 118 had been observed was retracted in 2001.

III

Naming of New Elements

For several years some international competition existed for naming these later entries in the periodic table. The International Union of Pure and Applied Chemistry (IUPAC), which oversees the naming of chemical entities, officially designates the permanent names given to these elements once their discoveries are confirmed.

Unconfirmed elements are known by temporary names determined by a system that uses the Latin words for the atomic number followed by the suffix -ium. The temporary name for element 116, for example, is ununhexium: un (one) + un (one) + hex (six) + ium.

IV

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 overcome 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. Some transuranium elements, such as plutonium 238, have been used as extremely compact and dependable sources of power. These sources convert radioactive-decay heat directly into electricity. Other transuranium isotopes, such as americium 241 and californium 252, have medical and industrial uses.

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