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