It has a hardness of 0.4. It oxidizes immediately on exposure to air and reacts violently with water , forming sodium hydroxide and hydrogen . The atomic weight of sodium is 22.99. Sodium is found in nature only in the combined state. Sodium is prepared commercially by the electrolytic decomposition of fused sodium chloride.
Sodium ranks seventh in order of abundance of the elements in the earth's crust.
It is a necessary constituent of plant and animal tissue. The element is used in the manufacture of tetraethyl lead and as a cooling agent in nuclear reactors. The most important compound of sodium is sodium chloride, known commonly as salt. Other important compounds include sodium carbonate, known as washing soda , and sodium bicarbonate, known as baking soda. Sodium hydroxide, known commercially as soda lye or caustic soda, is used in the manufacture of soap , rayon , and paper, in oil refining, and in the textile and rubber industries.