ChemGlobe: Sodium
   Electronic, thermal, and steric data along with basic information.
   http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/11.html

ChemicalElements.com: Sodium
   Basic information, atomic structure, and table of isotopes.
   http://www.chemicalelements.com/elements/na.html

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

It's Elemental: Sodium
   Basic physical and historical information.
   http://education.jlab.org/itselemental/ele011.html

LANL: Sodium
   Sources, properties, uses, and compounds.
   http://periodic.lanl.gov/elements/11.html

Lenntech: Sodium
   Physical data, chemical properties, health and environmental effects.
   http://www.lenntech.com/Periodic-chart-elements/Na-en.htm

Radiochemistry of Sodium
   Full text of the monograph by W. T. Mullins and G. W. Leddicotte (Oak Ridge National Laboratory, Oak Ridge, Tennessee). [PDF]
   http://lib-www.lanl.gov/cgi-bin/getfile?rc000010.pdf

Visual Elements: Sodium
   Image, animation, source, uses, key isotopes, and ionisation energies.
   http://www.chemsoc.org/viselements/pages/sodium.html

Wikipedia: Sodium
   Properties of the element, including its history, applications, and characteristics.
   http://en.wikipedia.org/wiki/Sodium