Thallium, a lustrous, bluish-white, metallic chemical element. Thallium is softer than lead and is malleable (capable of being hammered into a thin sheet). When thallium is in contact with air, a dull, blue-gray oxide coating forms; it protects the metal from corrosion.

Thallium and its compounds are toxic. Thallium sulfate, an extremely poisonous compound, is widely used in rodent and insect poisons. Other thallium compounds are used to increase the refractive power of optical glass and artificial gems (for example, diamonds), and to produce a bright-green flame for flares and fireworks. Because of its ability to resist corrosion, thallium is sometimes alloyed with copper or silver for making bearings.

Thallium was discovered in 1861 by the English scientist William Crookes. He coined the name from the Greek word thallos (young shoot or green twig) because of the brilliant green line in the element's spectrum. Thallium is widely distributed in the earth's crust. It is usually found in trace amounts, particularly as an impurity in iron and copper pyrites and in various zinc-bearing ores; larger amounts occur in the rare minerals crookesite and lorandite. Most thallium is recovered as a byproduct in the refining of zinc-bearing ores.

Symbol: Tl. Atomic number: 81. Atomic weight: 204.383. Specific gravity: 11.85. Melting point: 577 F. (303 C.). Boiling point: 2,655 F. (1,457 C.). Thallium has two stable isotopes: Tl-208 and Tl-205. It belongs to Group IIIA of the Periodic Table and can have a valence of +1 or +3.