Thallium: the essentials
When freshly exposed to air, thallium exhibits a metallic lustre, but soon develops a bluish-grey tinge, resembling lead in appearance. A heavy oxide builds up on thallium if left in air, and in the presence of water the hydroxide is formed. The metal is very soft and malleable. It can be cut with a knife.
The element and its compounds are toxic and should be handled carefully. Thallium may cause cancer.
Thallium: historical information
Thallium was discovered spectroscopically in 1861 by Crookes. The element was named after the green spectral line, which identified the element (Greek "thallos", green twig). The metal was isolated both by Crookes and Lamy in 1862. They had been expecting to isolate tellurium after removing selenium from the byproducts from a commercial sulphuric acid factory but instead foundthe new element thallium.
Thallium: physical properties
Thallium: orbital properties
Isolation: thallium metal would not normally be made in the laboratory as it is available commercially. Crude thallium is present as a component in flue dust along with arsenic, cadmium, indium, germanium, lead, nickel, selenium, tellurium, and zinc. This is done by dissolving in dilute acid, precipitating out lead sulphate, and then adding HCl to precipitate thallium chloride, TlCl. Further purification can be achieve by electrolysis of soluble thallium salts.