Thallium is a chemical element (atomic number 81). In it's pure state, it is a silvery grey metal that discolors quickly in the presence of oxygen. Although it is primarily used in electronics, it is also used in nuclear medicine. Both the metal and all of its compounds are highly toxic (see Heavy metal poisoning) and even exposure to the skin is problematic. Deaths from thallium poisoning are fairly common.

Thallium at Wikipedia

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