Thallium

Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray poor metal is not found free in nature. When isolated, it resembles tin, but discolors when exposed to air. Chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861, in residues of sulfuric acid production. Both used the newly developed method of flame spectroscopy, in which thallium produces a notable green spectral line. Thallium, from Greek θαλλός, thallos, meaning "a green shoot or twig," was named by Crookes. It was isolated by electrolysis a year later, by Lamy.
Symbol: Tl
Electron configuration: Xe 4f14 5d10 6s2 6p1
Atomic number: 81
Melting point: 303.8 °C
Discovered: 1861
Atomic mass: 204.3833 ± 0.0002 u