Thallium

Thallium is a soft, silvery-white post-transition metal that tarnishes to a bluish gray when exposed to air. It melts at a relatively low temperature and is highly toxic, especially in the form of soluble compounds. It is obtained mainly as a byproduct of refining metal sulfide ores and is used in electronics, infrared detectors, and specialty glass; a radioactive isotope is also used in some medical heart scans.