Thallium is a soft, malleable metal with the chemical symbol Tl and atomic number 81. It is part of the p-block of the periodic table and is known for its silvery appearance. Thallium is primarily used in electronics, optics, and as a component in certain types of glass. Discovered in 1861 by Sir William Crookes, thallium is toxic and can pose health risks if ingested or inhaled. It occurs naturally in trace amounts in some minerals and is often extracted as a byproduct of mining for other metals, such as lead and zinc.