Protactinium

ChemGlobe: Protactinium
Electronic, thermal, and steric data along with an isotope table.
http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/91.html

ChemicalElements.com: Protactinium
Basic information, atomic structure, and isotopes.
http://www.chemicalelements.com/elements/pa.html

EnvironmentalChemistry.com: Protactinium
Atomic structure, chemical and physical properties, and table of nuclides.
http://environmentalchemistry.com/yogi/periodic/Pa.html

It's Elemental: Protactinium
Basic physical and historical information.
http://education.jlab.org/itselemental/ele091.html

LANL: Protactinium
Basic information and history.
http://periodic.lanl.gov/elements/91.html

Visual Elements: Protactinium
General and physical information, source, and key isotopes.
http://www.chemsoc.org/viselements/pages/protactinium.html

WebElements: Protactinium
Extensive information on history, uses, occurrence, compounds, and properties.
http://www.webelements.com/webelements/elements/text/Pa/key.html

Radiochemistry of Protactinium
Full text of the monograph by H. W. Kirby (Mound Laboratory, Miamisburg, Ohio). [PDF] <small>(December, 1959)</small>
http://library.lanl.gov/cgi-bin/getfile?rc000046.pdf