Tl, or thallium, is a chemical element with the symbol Tl and atomic number 81. It is a soft, malleable metal that is grayish-white in color. Thallium is found in trace amounts in various minerals and is primarily used in electronics, optics, and as a component in certain types of glass. Thallium is known for its toxicity, which poses health risks if ingested or inhaled. Due to its harmful effects, safety precautions are essential when handling this element. In the past, thallium compounds were used in rat poisons and some medical treatments, but their use has declined significantly.