sodium

A soft, waxy, light, extremely malleable, silvery-white, unstable metallic element of the alkali group. It has the symbol Na in the periodic table and the atomic number 11. Sodium is the most common alkali metal and the sixth most abundant element on Earth. It appears only as a compound with other elements in nature. It has a strong affinity for oxygen and other nonmetallic elements. It is highly reactive, burns with a yellow flame, oxidizes in air, and reacts violently with water. Sodium provides the chief cation (positive ion) of extracellular body fluids. It is an essential nutrient in animals; it helps to maintain blood volume, regulate the balance of water in the cells, and keeps nerves functioning.