Thallium My element was Thallium. It is atomic number 81. It has 81 protons and electrons and 123 neutrons. Thallium has a mass of 204.3833 atomic mass units. Its symbol is Tl. It resides in Group IIIA of the periodic table. That is the aluminum family. Thallium has a bluish color after exposure to the air. It is a very soft and malleable metal. It has an electron configuration of 1s22s23s23p64s23d104p65s24d105p64f145d106s26p1. It has 6 electron shells. It melts at 576.7 K and boils at 1730 K. It is a solid when at room temperature. Thallium was discovered in 1861 by a British chemist and physicist. His name was Sir William Crookes. He discovered it spectroscopically in Engla