Thallium is a soft, heavy post-transition metal. The metal resembles tin when it's fresh, but discolors to blue-gray upon exposure to air. The element is soft enough to cut with a knife.
Description: Thallium is a toxic soft metal.
Where It's Used: Thallium is used to form low-melting point glass. It was once used as rat poison, but it is now banned from household use.