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(o͞on'o͞onŏk`tēəm), artificially produced radioactive chemical elementelement,
in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. A substance such as a compound can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved.
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; symbol Uuo; at. no. 118. Scientists from the Joint Institute for Nuclear Research in Dubna, Russia, and Lawrence Livermore National Laboratory in California collaborated in the discovery of ununoctium in experiments conducted in 2002 and 2005; the discovery was confirmed by the International Union of Pure and Applied Chemistry (IUPAC) in 2015. They bombarded atoms of californiumcalifornium
[from California], artificially produced, radioactive metallic chemical element; symbol Cf; at. no. 98; mass no. of most stable isotope 251; m.p. about 900°C;; b.p. about 1,470°C;; density unknown; valence +3.
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-249 with ions of calciumcalcium
[Lat.,=lime], metallic chemical element; symbol Ca; at. no. 20; at. wt. 40.078; m.p. about 839°C;; b.p. 1,484°C;; sp. gr. 1.55 at 20°C;; valence +2. Calcium is a malleable, ductile, silver-white, relatively soft metal with face-centered, cubic crystalline
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-48. Among the products of the bombardments were atoms of ununoctium-294 (one in 2002, two in 2005), each of which decayed into an atom of livermoriumlivermorium,
artificially produced radioactive chemical element; symbol Lv; at. no. 116; mass number of most stable isotope 292; m.p., b.p., sp. gr., and valence unknown.
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 by emitting an alpha particlealpha particle,
one of the three types of radiation resulting from natural radioactivity. Alpha radiation (or alpha rays) was distinguished and named by E. R. Rutherford in 1909, who found by measuring the charge and mass of alpha particles that they are the nuclei of ordinary
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. Uuo-294 has a half-life of approximately 0.89 msec. No name has yet been formally adopted for element 118, which is therefore called ununoctium, from the Latin roots un for one and oct for eight, under a convention for neutral temporary names proposed by IUPAC in 1980. In 2016 the name oganesson was proposed for the element, in honor of Russian physicist Yuri Oganessian.

In 1999 a research team at the Lawrence Berkeley National Laboratory in Calif. bombarded lead-208 atoms with high-energy krypton-86 ions to create what an analysis showed to be three atoms of element 118 with mass number 293 and a half-life of less than a millisecond. In 2001, however, the team retracted its claim to have produced ununoctium after other laboratories failed to reproduce their results and after a reanalysis of the original data did not show the production of element 118. A subsequent investigation suggested that the original finding was the result of fraud on the part of one of the team scientists.

See also synthetic elementssynthetic elements,
in chemistry, radioactive elements that were not discovered occurring in nature but as artificially produced isotopes. They are technetium (at. no. 43), which was the first element to be synthesized, promethium (at. no. 61), astatine (at. no.
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; transactinide elementstransactinide elements
, in chemistry, elements with atomic numbers greater than that of lawrencium (at. no. 103), the last member of the actinide series. See transuranium elements.
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; transuranium elementstransuranium elements,
in chemistry, radioactive elements with atomic numbers greater than that of uranium (at. no. 92). All the transuranium elements of the actinide series were discovered as synthetic radioactive isotopes at the Univ.
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References in periodicals archive ?
Numbered 113, 115, 117 and 118, the new elements have been given temporary working names of ununtrium, ununpentium, ununseptium, and ununoctium.
American and Russian scientists announced the creation of a new superheavy element, known as element 118 or ununoctium, which was generated by firing calcium ions at californium; three atoms resulted from the experiments, and each lasted a small fraction of a second.
9 US and Russian scientists working at Dubna in Russia have announced the discovery of chemical element ununoctium - what number element is it?