ChemGlobe: Potassium
   Electronic, thermal, and steric data along with basic information.
   http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/19.html

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

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

It's Elemental: Potassium
   Basic physical and historical information.
   http://education.jlab.org/itselemental/ele019.html

LANL: Potassium
   Sources, uses, properties, and isotopes.
   http://periodic.lanl.gov/elements/19.html

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

Radiochemistry of Potassium
   Full text of the monograph by William T. Mullins and G. W. Leddicotte (Oak Ridge National Laboratory, Oak Ridge, Tennessee). [PDF]
   http://lib-www.lanl.gov/cgi-bin/getfile?rc000016.pdf

Visual Elements: Potassium
   Image, animation, source, uses, physical information, and key isotopes.
   http://www.chemsoc.org/viselements/pages/potassium.html

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