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Sodium, symbol Na, and atomic number 11, is a soft, silvery-white alkali metal in the first group of the periodic table. It is highly reactive, especially with water, where it forms sodium hydroxide and hydrogen gas; due to this reactivity it is typically stored under oil or inert atmosphere and is never found as a free element in nature.

In nature, sodium is abundant in minerals such as halite (sodium chloride) and in seawater, where it exists mainly as dissolved ions. Industrial production of metallic sodium is achieved predominantly by the electrolysis of molten sodium chloride in chlor-alkali processes, historically using Downs-type cells. This method also yields chlorine gas and sodium hydroxide as major byproducts.

The only stable isotope of sodium is Na-23; many radioactive isotopes have been observed but are short-lived. The most important sodium compounds are sodium chloride (NaCl), used as table salt; sodium bicarbonate (NaHCO3), baking soda; sodium carbonate (Na2CO3), soda ash; and sodium hydroxide (NaOH), caustic soda. These compounds underpin a wide range of applications in food, chemical manufacturing, glass production, paper, and cleaning products. Sodium plays a vital role in biology, where it is essential for nerve function, muscle contraction, and maintaining fluid balance.

Historically, sodium was isolated by Humphry Davy in 1807 through the electrolysis of molten caustic soda. The name derives from natrium, Latin for soda, which traces to Greek terms for soda mineral sources.