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[ ? ] Actinide:
Radioactive and toxic.

Protactinium is a chemical element, having the chemical symbol Pa. Its atomic number (the number of protons) is 91. It has a standard atomic weight of 231.0359 g•mol −1 and it is a solid in its elemental form.

Protactinium is considered a member of the "Actinide" class of elements. At a pressure of 101.325 kPa, it has a boiling point of ~4,000 °C and a melting point of 1,572 °C.