Protactinium
Links
Radiochemistry of Protactinium
http://lib-www.lanl.gov/cgi-bin/getfile?rc000046.pdf
Full text of the monograph by H. W. Kirby (Mound Laboratory, Miamisburg, Ohio). [PDF]
It's Elemental: Protactinium
http://education.jlab.org/itselemental/ele091.html
Basic physical and historical information.
ChemicalElements.com: Protactinium
http://www.chemicalelements.com/elements/pa.html
Basic information, atomic structure, and isotopes.
Protactinium
http://www.scescape.net/~woods/elements/protactinium.html
Data tables and historic information.
Visual Elements: Protactinium
http://www.chemsoc.org/viselements/pages/protactinium.html
General and physical information, source, and key isotopes.
EnvironmentalChemistry.com: Protactinium
http://environmentalchemistry.com/yogi/periodic/Pa.html
Atomic structure, chemical and physical properties, and table of nuclides.
WebElements: Protactinium
http://www.webelements.com/webelements/elements/text/Pa/key.html
Extensive information on history, uses, occurrence, compounds, and properties.