A chemical element, symbol Na, atomic number 11, and atomic weight 22.9898. Sodium is between lithium and potassium in group I of the periodic table. The element is a soft, reactive, low-melting metal with a specific gravity of 0.97 at 20°C (68°F). Sodium is commercially the most important alkali metal. It was named by Humphry Davy, who first isolated it by electrolysis in 1807.