Thallium is a silvery-white post transition metal with a melting point of 304 degrees Celsius. It is extremely toxic, as with some of its neighboring elements. It has oxidation states of +1 and +3 with +1 being more stable. It is fairly reactive, and tarnishes quickly on exposure to air. It was discovered in 1861 by William Crookes. It has some similarities to Potassium which is what makes it so deadly. 

Thallium has few uses due to its toxicity. It used to be used as rat poison, but eventually you could no longer buy it due to its involvement in multiple murders. Today, it is used in electronics, photovoltaic cells, and in glass making.