ChemGlobe: Promethium - Electronic, thermal, and steric data along with an isotope table. - http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/61.html
LANL: Promethium - History, sources, uses, and properties. - http://periodic.lanl.gov/elements/61.html
Wikipedia: Promethium - Properties of the element, including its history, applications, and characteristics. - http://en.wikipedia.org/wiki/Promethium
Lenntech: Promethium - Physical data, chemical properties, health and environmental effects. - http://www.lenntech.com/Periodic-chart-elements/Pm-en.htm
EnvironmentalChemistry.com: Promethium - Atomic structure, chemical and physical properties, and table of nuclides. - http://environmentalchemistry.com/yogi/periodic/Pm.html