Thallium is a Block P, Group 13, Period 6 element. The number of electrons in each of Thallium's shells is 2, 8, 18, 32, 18, 3 and its electronic configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹. In its elemental form thallium's CAS number is 7440-28-0. The thallium atom has a radius of 170.pm and it's Van der Waals radius is 196.pm. Thallium and its compounds are highly toxic. Thallium is primarily used for its electrical conductivity as thallium sulfide which changes with exposure to infrared light. This ability makes the compound useful in photocells. Thallium bromide-iodide crystals have been used as infrared optical materials. Thallium has been used, with sulfur or selenium and arsenic, to produce low melting glasses which becomes fluid between 125 and 150 C. These glasses have properties at room temperatures similar to ordinary glasses and are said to be durable and insoluble in water. Thallium oxide has been used to produce glasses with a high index of refraction, and is also used in the manufacture of photo cells. Thallium is available as metal and compounds with purities from 99% to 99.9999% (ACS grade to ultra-high purity); metals in the form of foil, sputtering target, and rod, and compounds as submicron and nanopowder. Thallium was first discovered by Sir William Crookes in 1861. Thallium get its name from the Greek word "thallos" which means twig or green shoot. See Thallium research below.