(Chemical symbol, Be). A rare, lightweight, strong, brittle, toxic, bivalent metallic element which occurs in beryl and other silicates. It is alloyed with copper as an age-hardening agent and is used in aerospace structural material. Beryllium resembles magnesium in appearance and chemical properties. It is separated from its chloride by displacement with sodium. Atomic number, 4; atomic weight, 9.02; melting point, 1280°C (2336°F); specific gravity, 1.85.