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