thorium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A radioactive silvery-white metallic element that is recovered commercially from monazite. Its longest-lived isotope, the only one that occurs naturally, is Th-232 with a half-life of 14.1 billion years and is used as a nuclear fuel. Thorium is used in magnesium alloys, and its oxide is widely used in gas mantles of Welsbach burners. Atomic number 90; atomic weight 232.038; melting point 1,750°C; boiling point 4,788°C; specific gravity 11.72; valence 4. cross-reference: Periodic Table.

from The Century Dictionary.

• noun In 1900 Brauner announced his belief that, thorium, as generally known, is separable into two different elements. A little later Baskerville, working by a different method, came to the same conclusion, and in 1903 claimed to have effected its separation into three components, for two of which he proposed the new names carolinium and berzelium, retaining the name thorium for the third component. Later he found for this new or purified thorium an atomic weight of 220.1–220.6. In 1905 R. J. Meyer and A. Gumperz published the results of a careful revision of the previous work on this subject, which they found did not afford any evidence in support of the earlier claims of the separability of thorium. Thorium is a radioactive element and is the parent of a series of radioactive products of which eight separate members have already been identified. These are known as mesothorium 1, mesothorium 2, radiothorium, thorium X, thorium emanation, thorium A, thorium B, and thorium C, and are ordinarily present in thorium compounds. Thorium and its products appear to constitute a separate radioactive group or family, distinct from the uranium group, of which actinium, ionium, radium, and polonium are members, although generally found associated with it in minerals. Thorium itself emits only a-rays, but, owing to the presence of thorium disintegration-products, β- and γ-rays are also given out by thorium compounds.
• noun Chemical symbol, Th; atomic weight, 231.9. The metallic base of the earth thoria, discovered by Berzelius, in 1828, in a mineral from Norway, to which the name of thorite is now given, and which consists essentially of the silicate of thorium.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chem.) A metallic element found in certain rare minerals, as thorite, pyrochlore, monazite, etc., and isolated as an infusible gray metallic powder which burns in the air and forms thoria; -- formerly called also thorinum. Symbol Th. Atomic weight 232.0.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a chemical element (symbol Th) with atomic number 90.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a soft silvery-white tetravalent radioactive metallic element; isotope 232 is used as a power source in nuclear reactors; occurs in thorite and in monazite sands