The cover picture shows part of a computer-controlled automatic apparatus used for rapid separations of short-lived atomic nuclei by liquid chromatography. A programmable electronic control-unit (shown in the upper half of the picture) schedules the separation procedure. In the center of the picture the position is shown where the products of nuclear-chemical reactions are deposited and dissolved after transportation by a gas-jet. The solution is continuously fed onto either of the two chromatographic columns contained in the metallic heating blocks shown at the bottom of the picture. The flow of different elution media is controlled by pneumatic valves in the red cylinders. Such set-ups are used in the search for superheavy elements. More about the results of these experiments, about the syntheses of the heaviest chemical elements and about the future trends of such studies is reported by G. Herrmann on page 1417ff. (Photo: A. Zschau, GSI Darmstadt)
Review ArticlesElements 107, 108, and 109, the three heaviest artificial elements, were prepared by nuclear fusion in the Darmstadt heavy-ion accelerator UNILAC from the heaviest stable atomic nuclei, lead-208 and bismuth-209, and the neutron-richest stable isotopes of chromium and iron. The yields are extremely low; only three atoms of element 109 have so far been observed. All other transuranium elements are also products of nuclear-chemical syntheses. It has not yet been possible to detect the theoretically predicted "superheavy" elements with atomic numbers around 114 and ca. 184 neutrons: while they should be capable of existing, no route for their preparation has yet been devised.
Synthesis of the Heaviest Chemical Elements-Results and PerspectivesAromaticity and antiaromaticity have always been a subject of controversy. The donoracceptor concept has afforded new arguments for the understanding of these terms; donor and acceptor substituents in the same molecule stabilize cyclic compounds with (4n) n-electrons (see 1) and destabilize those with (4n + 2) n-electrons (see 2, which does not undergo aromatization). New compounds of this type are of interest, for example, for developments in the area of organic metals and in non-linear optics.
