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The symbol for the element thallium.
The symbol for thallium.
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.