A silvery, moderately hard alkaline-earth metal that constitutes approximately 36 percent of the earth's crust and is a basic component of most animals and plants It occurs naturally in limestone, gypsum, and fluorite, and its compounds are used to make plaster, quicklime, Portland cement, and metallurgic and electronic materials Atomic number 20; atomic weight 4008; melting point 842øC; boiling point 1,484øC; specific gravity 154; valence 2 cross-reference_ Periodic Table