Atomic Number
81
Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. It is highly toxic and was historically used in rat poisons and insecticides.
81
204.38 u
post transition
solid
11.85 g/cm³
2, 8, 18, 32, 18, 3
Protons: 81 • Electrons: 81 • Neutrons: Varies by isotope
Electron Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹
1861 • William Crookes