Thallium-203 Metal Isotope

See more Thallium products. Thallium (atomic symbol: Tl, atomic number: 81) is a Block P, Group 13, Period 6 element with an atomic weight of 204.38. The number of electrons in each of thallium's shells is 2, 8, 18, 32, 18, 3 and its electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹. The thallium atom has a radius of 170 pm and a Van der Waals radius of 196 pm. Thallium was discovered by Sir William Crookes in 1861 and first isolated by Claude-Auguste Lamy in 1862. Thallium is a post-transition metal that is not found free in nature. 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 also been used with sulfur, selenium or arsenic to produce low melting glasses which become fluid between 125 and 150 °C, while thallium oxide has been used to produce glasses with a high index of refraction, and is also used in the manufacture of photo cells. Its name is derived from the Greek word thallos, which means twig or green shoot.