Barium

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