thallium (thăl´ēəm), metallic chemical element; symbol Tl; at. no. 81; interval in which at. wt. ranges 204.382–204.385; m.p. 303.5°C; b.p. about 1,457°C; sp. gr. 11.85 at 20°C; valence +1 or +3. Thallium is a soft, malleable, lustrous silver-gray metal with a hexagonal close-packed crystalline structure. A member of Group 13 of the periodic table, it resembles aluminum in its chemical properties. In its physical properties it resembles lead. It forms univalent compounds similar to those of the alkali metals. It tarnishes rapidly in dry air, forming a heavy oxide coating; in moist air or water the hydroxide is formed. It dissolves in nitric or sulfuric acid. Thallium is widely distributed in nature, but the only minerals rich in the element are crooksite and lorandite. It is also found in copper pyrites and lead and zinc ores; it is recovered during the processing of these ores, the method of recovery depending on the source. Thallium is used in low-melting alloys with other metals and in compounds. Both the metal and its compounds are very poisonous. The sulfide is used as a rat poison and the sulfate as an insecticide. The oxide is used in special highly refractive optical glass. Several compounds are used in photoelectric cells and infrared detectors. Discovered spectroscopically in 1861 by Sir William Crookes, it was isolated independently by Crookes and C. A. Lamy in 1862.