thallium

/thal"ee euhm/, n. Chem.
a soft, malleable, rare, bluish-white metallic element: used in the manufacture of alloys and, in the form of its salts, in rodenticides. Symbol: Tl; at. wt.: 204.37; at. no.: 81; sp. gr.: 11.85 at 20°C.
[1860-65; < NL, equiv. to thall- ( < Gk thallós green stalk) + -ium -IUM; named after green line in its spectrum]

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 chemical element, metal of main Group 13 (IIIa, or boron group) of the periodic table, poisonous and of limited commercial value. Like lead, thallium is a soft, low-melting element of low tensile strength. Freshly cut thallium has a metallic lustre that dulls to bluish gray upon exposure to air. The metal continues to oxidize upon prolonged contact with air, generating a heavy nonprotective oxide crust. Thallium dissolves slowly in hydrochloric acid and dilute sulfuric acid and rapidly in nitric acid.

      Rarer than tin, thallium is concentrated in only a few minerals that have no commercial value. Trace amounts of thallium are present in sulfide ores of zinc and lead; in the roasting of these ores, the thallium becomes concentrated in the flue dusts, from which it is recovered.

      Sir William Crookes (Crookes, Sir William) discovered (1861) thallium by observing the prominent green spectral line generated by selenium-bearing pyrites that had been used in the manufacture of sulfuric acid. Crookes and Claude-Auguste Lamy independently isolated (1862) thallium, showing it to be a metal.

      Two crystalline forms of the element are known: close-packed hexagonal below about 230° C and body-centred cubic above. Natural thallium, the heaviest of the boron group elements, consists almost entirely of a mixture of two stable isotopes: thallium-203 (29.50 percent) and thallium-205 (70.50 percent). Traces of several short-lived isotopes occur as decay products in the three natural radioactive disintegration series: thallium-206 and thallium-210 (uranium series), thallium-208 (thorium series), and thallium-207 (actinium series).

      Thallium metal has no commercial use, and thallium compounds have no major commercial application, since thallous sulfate was largely replaced in the 1960s as a rodenticide and insecticide. Thallous compounds have a few limited uses. For example, mixed bromide-iodide crystals (TlBr and TlI) that transmit infrared light have been fabricated into lenses, windows, and prisms for infrared optical systems. The sulfide (Tl2S) has been employed as the essential component in a highly sensitive photoelectric cell and the oxysulfide in an infrared-sensitive photocell (thallofide cell). Also, the oxide (Tl2O) has been utilized as an ingredient in highly refractive optical glasses and as a colouring agent in artificial gems. Alkali halide crystals, such as sodium iodide, have been doped or activated by thallium compounds to produce inorganic phosphors for use in scintillation counters to detect radiation.

      Thallium imparts a brilliant green coloration to a bunsen flame. Thallous chromate, formula Tl2CrO4, is best used in the quantitative analysis of thallium, after any thallic ion, Tl3+, present in the sample has been reduced to the thallous state, Tl+.

      Thallium is typical of the Group 13 elements in having an s2p1 outer electron configuration. Promoting an electron from an s to a p orbital allows the element to be three or four covalent. With thallium, however, the energy required for sp promotion is high relative to the Tl–X covalent bond energy that is regained on formation of TlX3; hence, a derivative with a +3 oxidation state is not a very energetically favoured reaction product. Thus, thallium, unlike the other boron group elements, predominantly forms singly charged thallium salts having thallium in the +1 rather than the +3 oxidation state (the 6s2 electrons remain unused). It is the only element to form a stable singly charged cation with the outer electron configuration (n-1)d10ns2, which is, unusually enough, not an inert gas configuration. In water the colourless, more stable thallous ion, Tl+, resembles the heavier alkali metal ions and silver; the compounds of thallium in its +3 state are easily reduced to compounds of the metal in its +1 state.

      In its oxidation state of +3, thallium resembles aluminum, although the ion Tl3+ appears to be too large to form alums. The very close similarity in size of the singly charged thallium ion, Tl+, and the rubidium ion, Rb+, makes many Ti+ salts, such as the chromate, sulfate, nitrate, and halides, isomorphous (i.e., have an identical crystal structure) to the corresponding rubidium salts; also the ion Tl+ is able to replace the ion Rb+ in the alums. Thus, thallium does form an alum; but in doing so it replaces the M+ ion, rather than the expected metal atom M3+, in the formula M+M3+(SO4)2 · 12H2O.

      Soluble thallium compounds are toxic. The metal itself is changed to such compounds by contact with moist air or skin. Thallium poisoning, which may be fatal, causes nervous and gastrointestinal disorders and rapid loss of hair.

atomic number
81
atomic weight
204.37
melting point
303.5° C
boiling point
1,457° C
specific gravity
11.85 (20° C)
oxidation states
+1, +3
electronic config.
[Xe]4f 145d106s26p1

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Universalium. 2010.

Look at other dictionaries:

  • Thallium — (pronEng|ˈθæliəm) is a chemical element with the symbol Tl and atomic number 81. [ [http://periodic.lanl.gov/elements/81.html thallium ] , Los Alamos National Laboratory. Retrieved November 21, 2006.] This soft gray malleable poor metal resembles …   Wikipedia

  • Thallium — Mercure ← Thallium → Plomb Indium …   Wikipédia en Français

  • THALLIUM — Du grec thallos , verdure. Symbole chimique: Tl Numéro atomique: 81 Masse atomique: 204,37 g Point de fusion: 303,5 0C Point d’ébullition: 1 457 0C Densité (à 20 0C): 11,85 Métal mou, facilement fusible, facilement oxydable dans l’air, découvert …   Encyclopédie Universelle

  • thallium — Symbol: Tl Atomic number: 81 Atomic weight: 204.3833 Pure, unreacted thallium appears silvery white and exhibits a metallic lustre. Upon reacting with air, it begins to turn bluish grey and looks like lead. It is very malleable, and can be cut… …   Elements of periodic system

  • Thallium [1] — Thallium Tl, Atomgew. 204,1, weißes, dem Blei ähnliches Metall von 11,8 spez. Gew., Schmelzpunkt 290°. An der Luft erhitzt, oxydiert es sich, wird von verdünnter Schwefel und besonders Salpetersäure, nur schwer von Salzsäure gelöst. Es kommt in… …   Lexikon der gesamten Technik

  • Thallium — Thal li*um, n. [NL., fr. Gr. ? young or green shoot or branch, twig. So called from a characteristic bright green line in its spectrum.] (Chem.) A rare metallic element of the aluminium group found in some minerals, as certain pyrites, and also… …   The Collaborative International Dictionary of English

  • Thallĭum — Thallĭum, ein von Crookes 1861 u. Lamy 1862 entdecktes Metall; es gleicht am meisten dem Blei, ist etwas weniger weiß als Silber, mit einem Stich ins Bläulichgraue, auf frischem Schnitte von lebhaftem Glanze; es ist sehr weich u. dehnbar, läßt… …   Pierer's Universal-Lexikon

  • Thallĭum — Tl, Metall, findet sich mit Kupfer, Silber und Selen im Crookesit (16–18,5 Proz.) und Berzelianit, in geringer Menge in manchen Schwefelund Kupferkiesen, in Zinkblende, im Lepidolith, im Glimmer von Zinnwald, in Corandit, Carnallit, im Badesalz… …   Meyers Großes Konversations-Lexikon

  • Thallium [2] — Thallium, Tl, Atomgew. 204,0, spez. Gew. 11,85, Schmelzpunkt 302°, Siedepunkt bei 1500°. [798] Kommt als Lorandit (Tl As S2) auf dem Balkan vor. – Das Thalliumoxydulnitrat, TlNO3, wird wegen seines hohen spezifischen Gewichtes zur… …   Lexikon der gesamten Technik

  • Thallium — Thallĭum (chem. Zeichen Tl), durch Spektralanalyse von Crookes und Lamy entdecktes Metall, zinnweiß, kristallinisch, fast so weich wie Natrium, vom spez. Gewicht 11,8, Atomgewicht 204,1 und dem Schmelzpunkt 290°, findet sich in geringen Mengen in …   Kleines Konversations-Lexikon

  • thallium — rare metallic element, 1861, Modern Latin, from Gk. thallos young shoot, green branch (see THALLUS (Cf. thallus)) + element name ending ium. So called by its discoverer, Sir William Crookes (1832 1919), from the green line in its spectrum by… …   Etymology dictionary

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