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rare metallic element, 1861, Modern Latin, from Greek thallos "young shoot, green branch" (see thallus) + element name ending -ium. So called by its discoverer, Sir William Crookes (1832-1919), from the green line in its spectrum by which he detected it.
thallium thal·li·um (thāl'ē-əm)
A soft, malleable, highly toxic metallic element, used in photocells, infrared detectors, and low-melting glass. Atomic number 81; atomic weight 204.38; melting point 303.5°C; boiling point 1,473°C; specific gravity 11.85; valence 1, 3.
A soft, malleable, very poisonous metallic element that is used in photography, in making low-melting and highly refractive glass, and in treating skin infections. Atomic number 81; atomic weight 204.38; melting point 303.5°C; boiling point 1,457°C; specific gravity 11.85; valence 1, 3. See Periodic Table.
(Tl), chemical element, metal of main Group 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.