Thallium

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Thallium is a chemical element, having the chemical symbol Tl. Its atomic number (the number of protons) is 81. It has a standard atomic weight of 204.3833 g•mol ⁻¹ and is a solid in its elemental form.

Thallium is considered a member of the "Post-transition metal" class of elements. At a pressure of 101.325 kPa, it has a boiling point of 1,473 °C and a melting point of 304 °C.