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[ ? ] Lanthanide:

Promethium is a chemical element, having the chemical symbol Pm. Its atomic number (the number of protons) is 61. The standard atomic weight of 145Pm, its isotope with the longest half-life (17.7 years) is 144.9127 g•mol −1 and it is a solid in its elemental form.

Promethium is considered a member of the "Lanthanide" class of elements. At a pressure of 101.325 kPa, it has a boiling point of ~3000 °C and a melting point of 1,042 °C.