[en] All 17 artificially produced elements heavier than uranium were discovered by nuclear-chemical synthesis. The three heaviest ones - elements 107, 108 and 109 - were snythesized at the heavy-ion accelerator UNILAC in Darmstadt by nuclear fusion from the heaviest stable nuclei, lead-208 and bismuth-209, and the most neutron-rich stable isotopes of chromium and iron: element 107 from bismuth-209 (atomic number Z=83) and chromium-54 (Z=24), element 108 from lead-208 (Z=82) and iron-58 (Z=26), and element 109 from bismuth-209 and iron-58. The first isotopes detected were those with mass numbers 262 (Z=107), 265 (Z=108) and 266 (Z=109); these nuclei are short-lived α-emitters with half-lives of 8.2 ms, 1.8 ms and 3.4 ms, respectively. The yields of these reactions are extremely small; only three atoms of element 109 have ever been observed. Experiments to synthesize element 110 have given ambiguous results. All attempts to detect the 'superheavy' elements with proton numbers near Z=114 and neutron numbers near N=184 have thus far failed. These elements have been predicted theoretically, and many attempts to synthesize them have been made, e.g. at UNILAC by fusion of calcium-48 (Z=20) with curium-248 (Z=96), or by transference of protons in the collision of two very heavy nuclei such as uranium-238 (Z=92). Surprisingly, the heaviest known nuclei are far more stable toward spontaneous fission into two fragments than was expected, but their synthesis is strongly hindered - much more than was initially anticipated. It is this hindrance rather than the decreasing nuclear stability which seems to presently limit the extent of the periodic table: even heavier elements should be able to exist, but no way has yet been found to produce them. (orig.)