A high-melting, lightweight, corrosion-resistant, rigid, steel-gray metallic element used as an aerospace structural material, as a moderator and reflector in nuclear reactors, and in a copper alloy used for springs, electrical contacts, and nonsparking tools. Atomic number 4; atomic weight 9.0122; melting point 1,287°C; boiling point 2,470°C; specific gravity 1.848; valence 2. See Periodic Table.