Britannica Nobel Prize: Protactinium: Overview and basic data.
ChemGlobe: Protactinium: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Protactinium: Basic information, atomic structure, and isotopes.
EnvironmentalChemistry.com: Protactinium: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Protactinium: Basic physical and historical information.
LANL: Protactinium: Basic information and history.
Protactinium: Data tables and historic information.
Radiochemistry of Protactinium: Full text of the monograph by H. W. Kirby (Mound Laboratory, Miamisburg, Ohio). [PDF]
Visual Elements: Protactinium: General and physical information, source, and key isotopes.
WebElements: Protactinium: Extensive information on history, uses, occurrence, compounds, and properties.