ChemGlobe: Promethium: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Promethium: Basic information, atomic structure, and isotopes.
EnvironmentalChemistry.com: Promethium: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Promethium: Basic physical and historical information.
LANL: Promethium: History, sources, uses, and properties.
Lenntech: Promethium: Physical data, chemical properties, health and environmental effects.
Promethium: Data table and historic information.
Visual Elements: Promethium: Image, animation, source, uses, and ionisation energies.
WebElements: Promethium: Extensive information on history, uses, occurrence, compounds, and properties.
Wikipedia: Promethium: Properties of the element, including its history, applications, and characteristics.