Britannica Nobel Prize: Protactinium
   Overview and basic data.
   http://www.britannica.com/nobel/micro/483_52.html

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

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

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