A soft, silvery-white metallic element that is found in various minerals, often with other rare-earth elements, or is separated as a byproduct in the processing of certain uranium ores. It is used in materials for high-efficiency lighting, and an artificially produced radioactive isotope is used as a tracer in studies of oil wells and pipelines. Atomic number 21; atomic weight 44.956; melting point 1,541°C; boiling point 2,836°C; specific gravity 2.989 (25°C); valence 3. See Periodic Table.